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§ 2. The Clavicular Arch.

(i.) Since the clavicular arch was figured in PI. Hawkinsi (' Geol. Soc. Quart. Journ.,' 1874, p. 444), v. Zittel has figured the clavicular bones iu PI. laticeps (' Handbuch der Palaontologie,' vol. 3, p. 489); but, while the clavicles are clearly shown, the interclavicle is named episternum. The most important evidence of this strncture in PlesiosauridfB, however, is to be seen in Plesiomurus arcuatus (' Brit. Assoc. Rep.,' 1839, p. 76; and 'Cat. Toss. Kept, and Amph.,' Part II, p. 163), preserved in the British Museum. From that specimen, No. 2028*, the character has been attributed to Thaumatosaurus (loc. cit., p. 159): "Omosternum consisting of a large single plate, much expanded transversely, with a wide and shallow anterior notch."f The anterior margin of the interclavicle in this specimen resembles in contour that attributed to Eretmosaurus (' Geol. Soc. Quart. Journ.,' 1874, p. 445) in its wide open curvature; but there is no evidence to show whether the shoulder girdle, pelvis, and limbs in Plesiomurus arcuatus were constructed on the same plan as in PI. rugosus. There is no doubt that the bone consists of three distinct elements united by sutures. These are a median interclavicle and two lateral bones which I regard as clavicles. On the visceral aspect the triangular clavicles are separated from each other by the wide short posterior median bar of the interclavicle, but the clavicles extend forward so that only a narrow transverse bar of the tT-shaped interclavicle is exposed in front of them, extending across the entire width of the bone. The interclavicle is 10j inches wide, concave on its anterior margin, 1£ inch from front to back at the widened extremities of the cross-bar, and -fa inch in the same measurement towards the oblong middle portion of the bone. The ripht anterior transverse limb of the cross-bar is 4 inches wide; the left limb is 3 inches wide. The middle portion of the bone is 3^ inches wide and 2^ inches in anteroposterior measurement. The sutural line which defines the interclavicle is sagittal, and consequently irregular. On each side of this "T-shaped interclavicle (fig. 2), in contact with the posterior margin of Us transverse bar and the lateral margin of its short wide median stem, is a large triangular clavicle which is directed backward and outward. In harmony with the dimensions of the transverse bar, the right clavicle is the wider. Anteriorly it is 4f inches wide; it is nearly 6 inches long. The external border, which is slightly convex, is continuous with the truncated lateral termination of the interclavicle in front of it. These external margins diverge outward as they extend backward, so that the transverse measurement over the posterior extremities of the clavicles is 14| inches. The postero-internal

t Compare Sollau, ' Geol. Soc. Quart. Joum.,' vol. 37, 1881, p. 457.

contours of the clavicle are irregularly concave, and as they extend inward are continuous with the posterior border of the interclavicle,

[graphic]

Flos. 2 and 3.—Ventral and visceral aspects of clavicular arch of Plesiotaurua arcuatus, showing the median interclavicle and lateral clavicles. Ic ia placed on the anterior margin.

and as they extend outward approximate toward the external contour of the bone without meeting it posteriorly in a point.

The ventral aspect of the clavicular arch is different (fig. 3) owing to variation in the positions of the sutures between the bones. The interclavicle no longer shows the "[t-shaped contour of the visceral surface, but is a wide curved bar with an irregular sagittal termination on its postero-lateral extremities. This is owing to the method of its squamous interlocking with the clavicles, which overlap its visceral surface more in front, and overlap its ventral surface more behind, where their pointed extremities nearly meet each other in the median line behind the interclavicle, and in the inch of space from which they are absent there is a slight distortion of the bone, and some evidence of a median posterior notch. The triangular forms of the clavicles are more marked on this aspect of the bones than on the other.

The most remarkable character here shown is the squamous sutural interlocking of the three bones by which their shares in forming the clavicular arch is definitely established. It is also shown by different directions of the lines of growth in the clavicles and interclavicles.

An isolated clavicular arch in the British Museum, R. 1322, presents a similar charactor and form, and shows in its sutures similar evidence of composite character. It has been assigned to the species named Plesiosaurus megacephalus (Stutchbury) in the British Museum Catalogue. It has a similar resemblance to the anterior contour of the interclavicle in Eretmosaurus, but I am aware of no evidence by which the species is identified from this bone, beyond a general resemblance to some specimens in the Bristol Museum.

The correspondence of structure in these clavicular arches with that figured in Plesiosaurus Hawkinsi and Plesiosaurus laticeps is a coincidence of plan, though the difference may indicate a sub-genus, and shows, I submit, that the original definition of the bones was not a conjectural suggestion, as stated by Professor Sollas, but a recognition of sutures which separate the interclavicle from the clavicles. And it seems to me a sound induction that whenever the margins of the clavicular arch are concave in front and behind, those concavities border the interclavicle, and whenever there are wings produced outward and backward, as in the specimen now figured, those wings are formed by the clavicles in all Plesiosanridae.

(ii.) Sir R. Owen, in 1841 ('Brit. Assoc. Rep.,' p. 64), remarks on the shoulder girdle of Pliosaurus:—" The pectoral arch owes its chief strength to a pair of immensely expanded coracoids, having a broad and short entosternal bone on their anterior interspace, and supporting the clavicles or acromion productions of the scapulae."* Subse

* I have examined the specimens in the Museum of the University of Oxford quently ('Geol. Soc. Quart. Journ.,' 1883, p. 135) a diagram of the shoulder girdle in this genus was given by that author, which represents the scapula and coracoid as meeting each other on the Elasmosaurian plan; but, unlike Elasmosaurians, the scapulae are divided from each other on the visceral aspect by a long triangnlar interclavicle (named episternum) which shows a mesial notch in front. I have not seen this specimen, which is not assigned to any species, locality, or collection. It would appear to show an intermediate condition between Plesiosaurs and Elasmosaurs, but it is impossible for me at present to affirm this. No specimen is known to me which shows that in Pliosaiirus the scapula and coracoid completely enclose the coracoid foramina. The evidence is imperfect, but it leads to the conclusion that the shoulder girdle was Flesiosaurian in plan.

[graphic]

Fig. 4.—Interclavicle, Pliotaurus philarchui. ant-mar, anterior border; e, lateral surface which may have been a clavicular attachment.

Pliosaiirus philarchus, on which the genus Peloneustes has been founded (' Cat. Foss. Kept. and Amph.,' Part II), in form of the scapula closely resembles Pliosaurian remains in the British Museum. Their approximating margins are convex, and between those margins Mr. Lydekker has inserted the interclavicle (termed omosternum),

with Professor A. H. Green, F.R.S., without finding evidence of this entosternal part of the skeletou. What appear to be scapulae of Pliotaurus brachydeirus have the inner and outer borders of the bones sub-parallel, with the anterior extremity but slightly widened. Zittel has interchanged the names to Owen's figures of the shoulder girdles of Pliotaurus and PUtiotaurui. I have not seen the originals of those figures.

which is triangular, flat, very thin, and has perfectly straight sides, which, in their hinder approximating two-thirds, are slighly bevelled. There is no evidence given that the hone occupied the position which has been figured, and I see no reason for believing that it was not placed, as in other Sauropterygians, on the visceral surface of the slightly inclined scapulae, where there is a doubtful indication of what may be an imperfectly preserved right clavicle. If the straight lateral border of the interclavicle was in contact with the flat visceral surface of the scapula, the bones would be in harmonious relation. The bevelled margin appears to look inward, and is therefore inferred to have given attachment to a lateral ossification which was still more delicately thin. This condition is shown in the following figure of the bone.

(iii.) A third modification of the Plesiosaurian type may be* indicated by the specimen in the Leeds Collection in the British Museum numbered 36. It is small, and the bones are not sharply ossified and immature, as Mr. Leeds has always believed. But I have not observed any specimen in his collection which would, with certainty, represent its adult state. The bones of the shoulder girdle are thick, and the scapula and coracoid are formed on the Plesiosaurian type, in that the inner border of the scapula gives no evidence of a median precoracoid prolongation backward to meet the coracoid. There is no indication that the coracoids and scapulae ever met in the median line, even in the supposed adult condition, since there is no anterior median process to the coracoid; but there is a cartilaginous interval between them in front like that attributed to Pliosaurug. The scapula is a stout triradiate bone with a wide external process, and in form it resembles the bones attributed to Pliosaurus. But the cervical vertebrae have no trace of the Pliosaurian modification, and have the aspect of the vertebras of Plesiosaurus, except that the articulation for the rib is not divided in the cervical region. Some Plesiosaurs from the Lias have shown the closest possible approximation of those surfaces, but the divided condition of the rib facet did not terminate with the Lias species, since some specimens from the Wealden (which are referred to Cimoliosaurus, 'Brit. Mus. Cat. Foss. Kept.,' Part II, p. 227, No. 2,444, No. 26,000) retain the character in a condition similar to that attributed to Thaumatcsaurus carinatus {loc. cit., p. 168, fig. 57). It may be that the imperfect ossification causes the facet of bone to appear single in this Oxford Clay fossil, while its cartilaginous terminations during life may have been divided; but so far as the evidence goes it rather suggests a sub-generic modification of the genus Plesiosaurus as indicated by the scapular arch, distinguished by undivided articular heads to the cervical ribs, if the adult preserved

* I am not sure that this immature Plesiosaurian type did not, on attaining maturity, become tin- Elasmosaurian genus Cryptoclidui.

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