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XIII. On the Structure of the Jaws and Teeth of the Iguanodon.
By GIDEON ALGERNON MANTELL, Esq., LL.D.,

F.R.S., F.L.S., Vice-President of the Geological Society, &c.

Received May 25,-Read May 25, 1848.

IN the deltas and estuaries of rivers that are of great extent, and which flow through countries of varied geological structure, we naturally expect to find the remains of terrestrial vertebrated animals that have been transported by the currents from far-distant lands, in a more or less mutilated state; the skeletons broken up-the bones dissevered, fractured, and waterworn-the teeth detached from the jaws and dispersed— and all these separated parts promiscuously imbedded in the mud, silt, and sand of the delta, and intermingled with the debris of the flora of the country, and the durable remains of fishes, mollusks, and crustaceans, that inhabited the freshwater, or were denizens of the adjacent sea. Such is the condition in which the bones and teeth of oviparous quadrupeds are found in the Wealden formation of the south-east of England; and hence the difficulty of obtaining satisfactory evidence of the form and structure of the extinct reptiles whose relics are so abundant in some of these deposits.

To this cause may be ascribed the remarkable fact, that although several hundred teeth, belonging to seven or eight genera of Saurians, have been collected from these fluviatile strata, scarcely a portion of the cranium, and but a few fragments of the jaws, have been discovered. Every relic of this kind is consequently in the highest degree interesting, and it is therefore most gratifying to me to have it in my power to lay before the Royal Society a considerable portion of the lower jaw, with teeth, of an Iguanodon, recently obtained from a quarry near Cuckfield in Sussex; the locality in which, nearly thirty years since, I first discovered the teeth of this colossal herbivorous Lizard.

In the communication which I had the honour to address to this Society in 1841*, a fragment of the lower jaw of a Saurian was described as that of a young Iguanodon, and the anatomical considerations which led me to offer that interpretation were fully detailed. But although from the form and mode of implantation of the fangs of the mature teeth, and the position of the germs of the successional ones, this inference appeared to be highly probable, yet as none of the crowns of the teeth remained, the peculiar dental characters of the Iguanodon were absent, and the presumed generic identity could not be unequivocally established; since it was possible

* Philosophical Transactions, Part II. p. 131.

that the fossil might belong to the Hylæosaurus, or to some other genus of reptiles whose bones occur in the Wealden deposits.

The specimen to which I now solicit attention consists of nearly the entire dentary and coronoid bones of the right side of the lower jaw of an adult animal, retaining two successional teeth in place, and the germ of a third, with the alveoli or sockets of seventeen or eighteen mature molars. This fossil is the first indisputable portion of a jaw of the Iguanodon hitherto brought to light; although nearly a quarter of a century has elapsed since the publication in the Philosophical Transactions* of my first memoir on the teeth of this extinct reptile.

From the striking analogy presented by the worn teeth of the Iguanodon to the condition of the abraded molars in some of the large herbivorous Mammalia, the discovery of the maxillary organs of this reptile was an object of the highest palæontological interest, in order to determine in what manner the mastication of vegetables was effected by an animal belonging to a class, in which no living species is provided with jaws so constructed as to be capable of a triturating or grinding motion; nor with cheeks to retain the food while such a process is performed. And though from the absence of mature teeth in the sockets, and of the articular portion of the jaw, the proximal end being destroyed, the specimen before us does not afford a complete solution of the problem, yet it possesses characters sufficiently definite and intelligible to throw important light on the structure and functions of the dental organs of the Iguanodon; and it has also enabled me to ascertain the form of the upper jaw, from a portion of the left maxillary bone, collected many years since, and now in the British Museum, but whose peculiar characters I was previously unable satisfactorily to interpret.

Before entering upon the description of the highly interesting fossil which forms the principal subject of this memoir, I beg to express my most grateful acknowledgments to Captain LAMBART BRICKENDEN, of Warminglid, Sussex, by whom it was discovered, and skilfully extricated from the sandstone in which it was imbedded; and who, although I was personally unknown to him, in the true spirit of an ardent and liberal promoter of science, placed it at my disposal, as the original investigator of the fossil Saurians of the Wealden; a tribute of respect that I regard as a high reward for my humble efforts to advance those branches of natural knowledge, to which I have devoted the leisure moments of a life of professional toil.

The specimen when discovered was imbedded in a block of the fawn-coloured sandstone which occurs interstratified with beds of clay and limestone, throughout a considerable part of the Wealden districts of the south-east of England; fortunately this stone is not very compact, so that organic remains can be extricated from it by a skilful manipulator, with but little difficulty. This fossil, like most of the bones and teeth

* "Notice on the Iguanodon, a newly discovered Fossil Reptile, from the Sandstone of Tilgate Forest, in Sussex."-Philosophical Transactions for 1825.

† See CUVIER, Oss. Foss. tome v. p. 351.

found in the sandstone, is heavy, and of a rich umber colour, from impregnation with oxides of iron. It consists of the dentary, and part of the coronoid or complementary bone, of the right side, and is entire at the anterior part, but the posterior or opposite extremity is imperfect, probably to the extent of several inches. Its original relative position in the jaw will be understood by referring to my former communication*, in which the peculiar construction of the lower jaw in Saurian reptiles is described. In this place it may, however, be proper to remark, that of the six pieces on each side of which the inferior maxilla consists, that containing the teeth, and forming the anterior portion or symphysis, is termed the dentary; and the posterior part of this bone is united on the outside by suture to the complementary or coronoid, angular, and surangular; while on the mesial or inner side, it is covered below and behind the teeth by an expansion of the opercular bone.

The specimen is represented of the natural size in Plate XVI.; its dimensions are as follow:

Length from the front of the symphysis to the posterior extremity of the bone

inches.

21

Greatest width of the outer surface measured over the convexity, from the lower margin to the upper alveolar edge

69

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41

2

11

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Breadth from the anterior termination of the alveolar space across to the inner margin
Height of the alveolar parapet at the posterior part
Width of the alveolar space at the posterior part
Width of the alveolar space at the anterior part..

Length from the first anterior tooth to the symphysial extremity...

Height of the successional tooth (Plate XVI. fig. 1, b) 11⁄2 inch; greatest width 7.

5

The mesial or inner aspect of the fossil (Plate XVI. fig. 1) is flattened and smooth, and shows the successional teeth which remain in their original places (a, b), and the sockets (f,f,f) for nineteen or twenty teeth; the inner alveolar plate having been destroyed, and the mature molars dislodged, before the bone was imbedded in the rock. The deep conical groove or furrow, so constantly present on the inner side of the dentary bone in reptiles, and which from its being covered by the splenial or opercular piece, it may be convenient to designate the opercular furrow, is here entirely exposed (fig. 1, d, d) in consequence of the removal and destruction of that maxillary element. It is very large, and prolonged anteriorly (d) to within six inches of the symphysis; the opercular piece must therefore have more nearly corresponded with that of the Varanians or Monitors than with the Iguanas, in which it is of a rhomboidal figure, and relatively of limited extent. The lower margin of the jaw is thick and convex at the posterior part, and gradually becomes thinner towards the front, where it expands horizontally into a broad scoop-like process, which is terminated anteriorly by an obtuse projection or tubercle (Plate XVI. fig. 1, e); it thins out mesially to form the symphysial suture (s) that connects it with the opposite ramus. The upper margin is formed by the alveolar process, which has a thick external * See Philosophical Transactions for 1841, Plate V. figs. 3. 7.

parapet, deeply furrowed on the inner side, as seen in this view, by the sockets for the mature teeth (f,f). Strongly-defined ridges occupy the interspaces, and rising above the sockets produce a sharp crenated upper border. The alveolar space is protected on the inner side by a moderately strong plate or wall, which must originally have nearly equalled the outer parapet in height, but is now in a great measure broken away within this process the germs of the successional teeth were developed.

The mode of implantation of the teeth appears to have been intermediate between the pleurodont and thecodont types; for the teeth were not anchylosed to the alveolar wall as in the Iguanas, but free as in the Crocodiles; yet as the ridges that separate the dental sockets are smooth and rounded, it may be inferred that these were not rendered complete alveoli by transverse plates extending from the outer to the inner parapet, as is the case in the Megalosaurus*.

The dental sockets diminish in size, but somewhat irregularly, from the posterior to the anterior termination of the alveolar process; and the latter suffers a corresponding diminution in breadth, and terminates suddenly at the distance of five inches from the front. At this point the upper margin becomes attenuated and contracted in a vertical direction, and descending with a gentle curve, expands horizontally and mesially to unite at the symphysial suture (s) with the opposite ramus, the anterior part of the jaw being edentulous.

From the fortunate preservation of two successional teeth in their original position, the mode of dental development in the Iguanodon is clearly demonstrated. The coronal portion of the tooth was first formed, as seen in the germ, Plate XVI. fig. 1, a ; and the entire crown was completed (fig. 1, b) before the secretion of the shank or fang commenced, as in the existing Saurians. The formative pulp was situated in a distinct depression or cavity, on the inner side of the root of the tooth it was destined to supplant: this is obvious by the positions of the teeth above described; and also by the remains of a third germ towards the posterior part (fig. 1, c).

Although the peculiar characters of the molars of the Iguanodon were described somewhat in detail in my former communications, and the present fossil confirms in every essential particular the inferences suggested by the detached teeth, as stated in my memoir of 1825, yet several new and important points relating to the development and functions of the dental organs, are elucidated by the new acquisition which Capt. BRICKENDEN's researches have brought to light. The second tooth (Plate XVI. fig. 1, b) which occupies its natural position in the alveolar space, consists of the entire crown, having the serrated margin as perfect as in the recent state; and this is the first evidence I have obtained as to the mode in which the teeth were implanted. The flat enamelled front, characterized by its longitudinal ridges, is placed mesially, and parallel to, and within the inner alveolar wall; the smooth convex face filling up a depression in the outer parapet, in the interspace of two sockets of the mature molars. This position is the reverse of that in which the successional teeth in the Iguana are developed; for in that reptile the coronal germ occupies the same relative place as * See Dr. BUCKLAND's Bridgewater Essay, Plate 23.

in the mature state; namely, with the ridged face outwards, and the smooth side inwards, or towards the cavity of the mouth.

As the coronal portion of the tooth in the Iguanodon is not symmetrical, one lateral margin presenting a gentle curvature (Plate XVIII. fig. 4, a), and the other forming a broad angle at the base of the serrated border (Plate XVIII. fig. 4, b), the teeth belonging to one side of the lower jaw may readily be distinguished from those of the other; the lateral marginal angle (b) being always situated posteriorly. Guided by this character, Dr. MELVILLE and myself examined the numerous teeth in the British Museum and in my own collection, and were enabled readily to determine to which ramus any tooth belonged. Thus, for example, the specimen represented, Plate XVIII. fig. 4, which is the very counterpart of that implanted in the jaw (Plate XVI. fig. 1, b), is a perfect successional tooth, consisting of the crown before the formation of the fang, and belonging to the right side (a denotes the anterior, and b the posterior angle). The specimen, Plate XVIII. fig. 5, appertained to the opposite or left lower side, as is shown by the situation of the posterior marginal angle (b).

The position of the lower teeth in relation to the alveolar process, appears to have been somewhat changed during the upward growth of the coronal portion consequent on the progressive development of the fang; and it seems probable that the face of the crown became inclined rather obliquely forwards and outwards, and that the mature teeth were arranged in an imbricated manner. This opinion is supported by the form of the alveoli in the outer parapet, and the corresponding oblique curvature in the fangs of the mature molars, as shown in Plate XVIII. fig. 5; but this inference does not admit of that absolute proof which the perfect adaptation of a fullgrown tooth to one of the sockets would afford; for the alveoli are irregular, and none of the detached teeth in my possession will fit either of the sockets in the recently discovered dentary bone.

The situation of the germ in relation to the tooth it was destined to supplant, is invariably on the inside of the mouth; in the lower molars the excavation in the mature tooth occasioned by the upward growth of the germ, is consequently on the enamelled mesial face, as is shown in my original memoir*: in the upper tooth the germ was lodged in an excavation on the smooth convex aspect, as will subsequently be demonstrated (see Plate XVIII. fig. 2", f).

In the fossil represented in the annexed sketch, the cavity produced by the pressure of the germ is situated in the fang of the tooth in place; in other examples, however, the successional dental excavation is on the base of the enamelled crown; for in the Iguanodon the old teeth were retained till nearly the entire coronal portion was worn away, and the crown of the tooth, from the abrasion by use above, and the removal of the fang by absorption below, was reduced to a mere disk, before it was finally shed; as in the specimen (figured in Xylograph, No. 2, p. 188). * Philosophical Transactions, 1825, Plate XIV. fig. 7 a.

[blocks in formation]

Xylograph, No. 1.

[graphic]

Posterior marginal angle.

Lower tooth of the right side.

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