SJcehton of a Inrgs Plesiosaur. 413 



(levclopnd anteriorly hikI posteriorly, so that tlic median loi)^ 

 axis of the hone divides it cqnally ; it is also smaller than 

 in the hnmerns. The head, trochanter, and nmscle-impre.s- 

 sions are well developed. The rest o£ the hind paddle is 

 unknown. 



As already noticed, this PIcsiosanr seems to resemble most 

 nearly that of which the skeleton is described and rather 

 badly figured by Carte and Daily* under the name of 

 P/esinsaiiriis cramptoni. It is very unfortunate that this 

 fine skeleton has never been properly prepared and developed, 

 so that the shoulder and pelvic girdles are almost completely 

 hidden in matrix and consequently are not available for 

 comparison with the present specimen. The general pro- 

 ])ortions of the skull and vertebral column seem to be much 

 the same in both, but there are several differences which 

 indicate that the two are not specifically identical. Thus in 

 the skull of our specimen the external nasal openings are 

 situated considerably in front of the orbits, while in 

 P. cramptoni they are scarcely at all in advance of them. 

 Again, the form of the platform of the neural arch in the 

 cervical vertehraj, with their nearly horizontal zygapophyses, 

 is very different from that of the cervical vertebra figured by 

 Carte and Baily, in which the zygapophysial surfaces are 

 strongly inclined. Furthermore, in the N(jrtliamptou speci- 

 men the humerus is relatively considerably larger than in 

 P. cramptoni, and its distal extremity is more expanded. 

 It seems therefore that our specimen should be regarded as 

 at least specifically diff^ercnt from P. a-amptoni. This species 

 was referred by Prof. II. G. Seeleyf to a distinct genus, 

 li/iotna/eosaurus, giving, however, somewhat inadecjuate 

 reasons for this. I propose to adopt the generic name 

 Rhomolt'osaurus, and define the genus as follows : — 



Plesiosaurs with a relatively large head and short neck 

 (the proportions being as five to eight). Cervical vertebrie 

 with very short centra and a divided rib-facet, ^^'ell-deve- 

 lopcd sacrum. Shoulder-girdle with strongly developed 

 clavicular arch with hroadly concave anterior border ; 

 coracoids short in post-glenoid region. Pelvis with a com- 

 l)aratively short pubis, an elongated oval obturator foramen, 

 and a posteriorly truncated ischium. The present species I 

 projjose to call Rhoinalcosaurus thorntoni, sp. n., in honour 

 of II, Gerard Thornton, Esq., who collected the remains 

 and presented them to the British Museum. 



* Loc. cit. supra. 



t Quart. Journ. Geol. Soc. vol. xxx. p. 418 (1874). 



