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sunted 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 Flesiosaurus, bnt the clavicles are much elongated. They form a squamous overlap on the visceral surface of tho 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 Flesiosaurus, 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 Flesiosaurus. It would therefore seem to follow that the precoracoid foramen of Nothosaurus becomes the coraco-scapular foramen of Flesiosaurus, 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 Flesiosaurus. Nevertheless, since the precoracoids meet in the median line in many Amphibians, and in Chelonians, and the scapula; 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 scapulae 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 Dieynodon 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 scapulas extended inward and backward at its expense; so that, while the scapulae are conveniently so named, it may be recognised that in Elasmosaurus, Colymbosaurtts, Murcenosaurus, 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 scapulae with the coracoids. But since the Plesiosauridas 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 seapulte 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 A rch.
Early writers regarded the median bone anterior to the coracoids in Plesiosaurus as the sternum. Sir R. Owen named it episternum. Profeasor 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 HawMnsi, and drew attention to the similar condition in PI. 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 arc 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 genns Catamites 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 Nothosanria 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 scapulae 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 "]"- or Vshaped median piece flanked by separate lateral ossifications as in Plesiosaurus, while this condition parallels the interclavicle and clavicles in all animals in which they are found.
It has never been shown thatjany one of the bones in question in Plesiosaurus retains a surface which has the aspect of having been cartilaginous. On the contrary, every specimen which I have examined is more or less thin and squamous, with contours completely ossified to sharp edges, even in the most immature specimens; while the interclavicle, when preserved, unites with the clavicles either by a thin squamous overlap or by sagittal sutures. This condition seems to me to demonstrate that the bones are membrane bones. I submit it follows that they are clavicles, and therefore that the visceral position of the clavicular arch, although anomalous, is not inconsistent with clavicular homology. Bone for bone, the three clavicles in Plesiosaurus seem to me to correspond to those of Ichthyosaurus and Nothosaurus. In the former their union is usually squamous, in the latter it is Natural. In Sauropterygia both conditions are found. The proposal made to identify the three anterior bones in the shoulder girdle in Nothosaurus as omosternum and precoracoids introduces the precoracoid as a distinct bone,* which is not known to be paralleled in any allied group of animals except the Anodomontia, in which there is no omosternum, and where the precoracoids are differently conditioned, being in the closest union with the coracoids, with a well-developed clavicular arch. But when the supposed precoracoids of Nothosaurus are recognised as clavicles, which rest by squamous overlap on the visceral surfaces of the scapulae, like the clavicles of Plesiosaurus, the clavicular arch is in harmony with that of the Sauropterygia, and the supposed differences in its composition disappear.
There are two family types in the Sauropterygia defined by differences in the shoulder girdle and other characters, known as Plesiosauridae and Elasmosauridae, though the organic differences which characterise them have not been fully set forth.
II. Further Evidence Of The Nature Op The Clavicular Arch
IN THE Plesiosaurid*.
§ 1. Nature and Limits of the Family.
There are four principal genera of Plesiosauridae, which are named Plesiosaurus, Eretmosaurus, Khomaleosaurus, and Pliosaurus. The family is characterised by the cervical ribs being attached to the vertebra by more or less completely-defined double facets and by the scapula being separated in the median line by the clavicular arch, by
* Uulke, loc. cit.
which they are braced to the coracoids. In the British Museum Catalogue (' Fossil Kept, and Amph.,' Part II), the Plesiosauridae is made to also include the Elasmosauridae, and the genera are enumerated in the following order:—Pliosaurus, Peloneustes, Thaumatosaurus, Polyptychodon, Cimoliosaurus, Eretmosaurus, Plesiosaurus. I should restrict the family to the fossils indicated by the names Pliosaurus, Peloneustes, Thaumatosaurus, Eretmosaurus, and Plesiosaurus. Good skeletons of these genera are known with the exception of Thaumatosaurus, which was founded by von Meyer (' Palaeoutographica,' vol. 6) upon remains which closely resemble those of Pliosaurus. And, after examining the type specimens, which are imperfect cervical vertebrae, dorsal vertebrae, teeth, and portions of the hinder region of the maxillary bone, I was unable to discover any character inconsistent with reference of the species to Pliosaurus. The head was evidently as large as in Pliosaurus; the teeth are circular in the crown, and show no trace of the area more or less flattened and free from carination defined by a lateral ridge on each side which characterises the anterior teeth of Pliosaurus grandis, resembling in this respect the posterior teeth. In the late cervical vertebra figured by von Meyer, the centrum has the same form and relative shortness from front to back as in Pliosaurus; the articular facet for the rib is similarly elevated, has a like transverse division forming a superior subtriangular part and an inferior transversely ovate part. The only characters in which there is not absolute agreement with the English species are that the articular faces of the centrums are more circular and more concave. These differences may be of specific value; and von Meyer's species may be classed as Pliosaurus oolithicus, till it is fully known. For similar reasons I am unable to separate Peloneustes from Pliosaurus. And if the type species was originally referred to Plesiosaurus,* it was because I then regarded the subtriangular crowns of anterior teeth in Pliosaurus as a generic character, and that character now seems less important. It has been necessary thus to explain differences of nomenclature, because the genus Thaumatosaurus ('Brit. Mus. Cat. Foss. Rept.,' Part II) has been made to include six species in addition to the type, which, with one exception, are all from the Lias. They were previously named Rhomaleosaurus Cramptoni, Plesiosaurus arcuatus, P. megacephalus, P. carinatus, P. propinquus, P. indicus. I am unable to place any of these species in Pliosaurus or Thaumatosaurus, nor is there evidence that all are referable to one genus; and it does not appear that a genus based on characters drawn from this assemblage of species can displace the definite conception of von Meyer indicated in the type of Thaumatosaurus. Most of these species not included in Rhomaleosaurus appear to belong to Eretmosaurus.
• 'Index to Aves, Omith., and Bept. in Woodw. Mus.,' 1869, p. 139.