and Clavicular Arch in Sauropterygia. 



127 



which they are braced to the coracoids. In the British Museum 

 Catalogue ('Fossil Rept. and Amph.,' Part II), the Plesiosauridse is 

 made to also include the Elasmosauridse, and the genera are enume- 

 rated in the following order : — Pliosaurus, Peloneustes, Thaumato- 

 saurus, Polyptychodon, Cimoliosaurus, Eretmosaurus, Plesiosaurus. I 

 should restrict the family to the fossils indicated by the names Plio- 

 saurus, Peloneustes, Thaumatosaurus, Eretmosaurus, and Plesiosaurus. 

 Good skeletons of these genera are known with the exception of 

 TTiaumatosaurus, which was founded by von Meyer (' Palaeontograph- 

 ica,' 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 teefch. 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 sub- 

 triangular 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 cha- 

 racter now seems less important. It has been necessary thus to 

 explain differences of nomenclature, because the genus Thaumato- 

 saurus ('Brit. Mus. Cat. Foss. Kept.,' Part II) has been made to in- 

 clude six species in addition to the type, which, with one exception, 

 are all from the Lias. They were previously named Bhomaleosaurus 

 Gramptoni, Plesiosaurus arcuatus, P. nrnegacephalus, 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 Thaumato- 

 saurus. Most of these species not included in Bhomaleosaurus appear 

 to belong to Eretmosaurus. 



* ' Index to Aves, Ornith., and Eept. in Woodw. Mus.,' 1869, p. 139. 



