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these three bones terminates in a sharp edge or point, and no one of them shows a terminal surface which has the aspect of being cartilaginous; there is therefore no evidence of a cartilaginous origin, while the mutual relations of the bones and their forms closely parallel the clavicles in Chelonians and some Lacertilians; they are therefore identified with those elements,* because there is no ground for regarding them as parts of the sternum.

§ 2. In Sauropterygia.

The nomenclature of the bones of the shoulder girdle in the Sauropterygia has given rise to greater differences of opinion.† The Amphibian mode of ossification of the long bones by long conical epiphyses connected by a dice-box shaped shaft, coupled with a general resemblance of the pelvis and shoulder girdle to those of Chelonians, led to the conclusion that, while the scapula, precoracoid, and coracoid are all separate from each other in some existing Amphibians, the Amphibian plan had been modified in the Chelonia by the scapula and precoracoid remaining undivided, while in the Sauropterygia the coracoid and precoracoid remained in primitive undivided condition. The strongest argument against this is the absence of a recognisable coracoid foramen in Sauropterygia in the coracoid bone,§ which, when present, may be supposed always to occur at the junction of the coracoid and precoracoid elements. At the same time the clavicles were recognised in the Sauropterygia as distinct lateral ossifications, placed at the sides of the interclavicle, and closely united with it by suture in some species of Plesiosaurus.

The difficulty with the scapular arch is in the theoretical postulates concerning the elements of which its several parts consist. Sir R. Owen ('Brit. Assoc. Rep.,' 1839, p. 56) used the Chelonian hypothesis to explain the scapula in Plesiosaurus. And E. D. Cope (Amer. Phil. Soc. Trans.,' 1870, p. 51), in Elasmosaurus, regarded the ventral plate of the bone anterior to the coracoid as clavicles or procoracoids, assuming a scapula external to it, as in Chelonians. And in 1883 Mr. Hulke (loc. cit.) also urged that the Plesiosaurian bone, usually named scapula, is a compound bone formed of scapula and precoracoid, as in Chelonians.

In support of this interpretation, it might be urged that the bones

* Hulke, loc. cit.

Those which prevailed prior to 1874 were summarised in the 'Quart. Journ Geol. Soc.,' for that year.

Index to Fossil Rept. Woodw. Mus.,' pp. 98, 120, 1869.

§ The specimen figured 'Quart. Journ. Geol. Soc.,' 1874, p. 446, is suggestive, but not conclusive, for the bone is very thin, and no portion of the margin of the perforation is seen, so that its existence as a normal character is unproved.

are similarly situate in both ordinal types, and similar in having a scapular portion which extends vertically, and a precoracoid portion which extends inward horizontally to the median line. On the other hand, there is no close resemblance between these Orders in the bones in question. (1.) The coracoids are dissimilar in form, and are differently conditioned, for they do not meet in the median line in any Chelonian, while there is no Sauropterygian in which they have not a mesial union. (2.) In Mr. Hulke's figures it is only the anterior portion of the ventral plate of the scapula which is lettered as precoracoid. Thus the precoracoid does not enter into the humeral articulation, or hold any position which theoretically can be compared with the bone in Chelonia; while the clavicular arch is anterior to the supposed precoracoid in Sauropterygians, but holds no comparable relation in Chelonians. This latter difficulty apparently led Mr. Hulke to regard the bones termed clavicular as omosternal in Sauropterygia. But no Chelonian possesses an omosternum; so that, if the identification were demonstrated, it does not support the Chelonian hypothesis of the shoulder girdle.

First, it may be observed that it is only in Anura that the precoracoid enters the anterior margin of the glenoid cavity; but in Urodela the precoracoid appears to be excluded, so that it is not theoretically impossible on an Amphibian hypothesis for the precoracoid to be anterior to the acetabulum; but the bone is always wedged between the scapula and coracoid, and on the coracoid border the coracoid foramen is always persistent, so that there is no analogy between Urodele and Sauropterygian to sustain the identification of the precoracoid which has been offered. Whenever two divergent bones form the scapular arch those two bones are the coracoid and scapula; but there is no analogy to support the hypothesis that the precoracoid might form the free extremity of the scapula, as in Mr. Hulke's figure (loc. cit.). There is no conclusive evidence of the mutual relations of the scapulo-precoracoid to the glenoid cavity in the Chelonia, but, unless it could be shown that the relations of these bones to the shoulder girdle were the same in both types, Chelonian analogy with Sauropterygia in this part of the skeleton rests upon an inconclusive basis of fact. In Chelonians the ascending process of the scapula extends dorsally towards the vertebræ, while in Sauropterygia it extends backward above the glenoid articulation for the humerus, and there is no evidence that these structures are homologous.

If the evidence is insufficient to sustain the interpretation discussed, it is found that the precoracoid has disappeared as a separate element from the skeleton in Lacertilia, and in most existing Ornithomorpha (Roy. Soc. Proc.,' vol. 49, p. 520). It is recognised in association with the coracoid in certain Birds; and the persistence of the coracoid foramen gives some evidence that the precoracoid is not unrepre

sented on the scapular side of the foramen in all members of that class except the Ornithosauria. Its individuality is retained in some of the Sauromorpha; and, although they have no distinct osseous representative of the bone, the nearest analogy to the shoulder girdle in Sauropterygia is found among Nothosauria; and there is no doubt that these resemblances and those with Anomodonts are closer than with existing orders of animals.

The Nothosaurian shoulder girdle contains the same number of constituent elements as in Sauropterygians, and the same nomenclature has been applied to them. There are some slight differences in the coracoid. In the Nothosauria it lies more obviously behind the glenoid cavity, while in many Plesiosaurs, especially the typical forms from the Lias, it also has a considerable median anterior extension. Further, in Nothosaurs there is a notch in the anterior margin of the coracoid, already contrasted with the similar notch in the coracoid of Ichthyosaurs, anterior to which are rough cartilaginous surfaces of scapula and coracoid, which have the aspect of having supported a cartilage which completed the coracoid foramen. There is no anterior prolongation of the scapula in Nothosaurus such as is seen in Plesiosaurus, but the clavicles are much elongated. They form a squamous overlap on the visceral surface of the scapula, according to von Meyer, and their length, and prolongation forward, removes the interclavicle from contact with the coracoids. If the suggested precoracoid cartilage in Nothosaurus existed, it makes the nature of both coracoid and scapula clearer in Plesiosaurus, and shows that the precoracoid need not be displaced into the position here assigned to the clavicle.

First, the foramen which appears to be indicated in the anterior margin of the coracoid in some species of Ichthyosaurus as a deep narrow notch, in other species widens to a concave anterior border to the bone; and similarly, in the specimen figured by Deecke as Lariosaurus Balsami, there is no trace of the anterior notch in the coracoid such as characterises Nothosaurus, but that bone has a smooth sharp anterior concave border such as the bone shows in Plesiosaurus. It would therefore seem to follow that the precoracoid foramen of Nothosaurus becomes the coraco-scapular foramen of Plesiosaurus, and that the precoracoid in Elasmosaurs ceases to exist as a distinct cartilage. It cannot be inferred to be lost by connation with the coracoid, because the foramen might then be supposed to persist, but, as there is no foramen in either the scapula or coracoid,* there is no evidence of the composite nature of either bone in Plesiosaurus. Nevertheless, since the precoracoids meet in the median line in many Amphibians, and in Chelonians, and the scapulæ never have a median ventral union, there is an a priori probability that bones formed from cartilage, placed *Always subject to the doubtful evidence of the Brit. Mus. fossil 2011*.

anterior to the coracoids, meeting in the median line, should rather be precoracoids than scapulæ in such Sauropterygia as show these characters. It has already been shown to be probable that the foramen anterior to the coracoid is the precoracoid foramen, having undergone such an enlargement in transition from Nothosaurus to Plesiosaurus as does the obturator foramen between the pubis and ischium in transition from the pelvis of Dicynodon to the pelvis of a Mammal. Therefore the precoracoids may have ceased to be differentiated, even as separate cartilages, and the coracoids may have grown forward at the expense of this cartilage, just as the scapulæ extended inward and backward at its expense; so that, while the scapulæ are conveniently so named, it may be recognised that in Elasmosaurus, Colymbosaurus, Muranosaurus, and their allies, the parts of the bone which meet in the median line, and are in median contact with the clavicular arch, are theoretically in the position of precoracoid elements, which connect the scapula with the coracoids. But since the Plesiosaurida show no such median union of scapular elements, or ossifications in front of the coracoids, it follows that there is no evidence that the precoracoid was ossified at all, while the cartilage representing it, if present, must have been a slender bar, comparable to the suggested precoracoid cartilage in Ichthyosaurus, as shown by the absence of a thick cartilaginous truncation of the anterior median termination of the coracoids in Plesiosaurus.

In the Anomodontia the plan of development of the shoulder girdle has been modified by the great extension of the clavicular arch outward and upward, so that the scapulæ are rather on the type of the Ichthyosauria than of the Sauropterygia. But the position and relations of the Anomodont precoracoid furnish some support to the interpretation given to the element in Ichthyosauria and Sauropterygia; because, if the precoracoid foramen in Anomodonts were theoretically enlarged to the dimensions seen in Colymbosaurus, Plesiosaurus, or Lariosaurus, it would be manifest that for so long as it connected the scapula and coracoid it was Elasmosaurian; so long as it remained attached to the extremity of the scapula only it would be Plesiosaurian; and so long as a remnant remained of cartilage in contact with the inner border of the clavicle the condition would be Lariosaurian.

There is thus a fundamental difference of plan between the imperforate coracoid of Sauromorpha and the perforate coracoid of Ornithomorpha, which depends upon the way in which the precoracoid bone loses its individuality.

§ 3. Nomenclature of the Bones in the Clavicular Arch.

Early writers regarded the median bone anterior to the coracoids in Plesiosaurus as the sternum. Sir R. Owen named it episternum. Pro

fessor Huxley regarded it as interclavicle and clavicles (Anatomy of Vertebrated Animals,' 1874, p. 210). In 1874 I figured the clavicles as posterior to the interclavicle in Plesiosaurus Hawkinsi, and drew attention to the similar condition in Pl. laticeps (Geol. Soc. Quart. Journ.,' 1874, p. 444, since figured by Zittel). Mr. Hulke, in 1883 ("Presidential Address, Geol. Soc.," p. 20), regards these ossifications as indivisible, and names the mass omosternum, thus reverting to the hypothesis that the ossifications have a cartilaginous origin, and are episternal. It follows from Mr. Hulke's views that the reputed clavicles of Nothosaurus are precoracoid, and the median bone between them is the omosternum.

The late Professor W. K. Parker fully discussed the omosternum in the Vertebrata. It is found in Mammals and in Anura, but is not present in all Anura, and is not always ossified. In the genus Calamites it appears to extend slightly on the visceral surface of the precoracoids. In the Amphibian group which it characterises clavicles are probably not found, so that it is in place of an interclavicle, if it does not represent it. It is sometimes single, sometimes paired, but never tripartite, as the median bone among Sauropterygians. Among Mammals Mr. Parker found the omosternum (paired) uniting with the sternum, while laterally it is continued by the clavicles, though there is a pair of small cartilages, termed precoracoids, between it and those elements of the skeleton. In the Monotremata the interclavicle is in the position of the omosternum. In Anguis fragilis Mr. W. K. Parker figures both interclavicle and clavicles, but there is no omosternum. The omosternum behaves as though it were the name given to the interclavicle when that element ossifies from cartilage.

A sternum is developed in every existing animal in which the omosternum is present, but in no Sauropterygian is there ever any trace of a sternum, so that there is nothing to suggest an omosternum. The omosternum has not been recognised in any existing order of Reptiles, and the Sauropterygia is the only fossil type except the Nothosauria in which it has been supposed to be found. That suggestion appears to rest upon the fact that the omosternum is found anterior to the precoracoids in certain existing Amphibia. There is the circumstance that the bones in Plesiosaurus extend on the visceral surfaces of the scapula and coracoids, while the clavicles in Ichthyosaurus are on the anterior and ventral surfaces of the same bones; but no animal is known in which the omosternum is developed in the position of the bone which has been so named in Plesiosaurus, and, so far as position goes, there is no evidence known to me which suggests that the bones in question should be omosternal rather than clavicular.

The omosternum has never been shown to consist of a T- or V

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