m tlEPORT~-1839. 



length and breadth, and its bent or reniform figure, and parti- 

 cularly with respect to the curvature of its outer margin, de- 

 viates in a greater degree than the ulna from the corresponding- 

 bone in the PL dolichodeirus. The differential characters af- 

 forded by the bones of the fore arm and leg are the more satis- 

 factory, because, as we shall presently see, the Fl. macroce- 

 phalus again presents different and characteristic forms of the 

 same bones. There are other and slighter differences in the 

 shape of the hatchet-bones, or cervical ribs, of the humerus and 

 of the femur. 



The length of the skeleton described by Mr. Conybeare is 

 nearly ten feet. 



Localities. — The most common places of deposit of the bones 

 of tliis species are in the lias of Somersetshire at Watchett, 

 Bath and Bristol ; and in that of the valley of Lyme in Dorset- 

 shire. I have likewise seen detached vertebras of the PI. doli- 

 chodeirus from the lias of Bitton in Gloucestershire. 



Plesiosaiirus macrocephalus. 



The characters of this interesting species I have fortunately 

 been able to study, not only in detached bones in different col- 

 lections, but also in an almost entire specimen liberally placed 

 at my disposal for that purpose by Viscount Cole. 



As, however, only a portion of the tail is preserved in this 

 unique specimen, the total number of vertebrae characteristic of 

 the PL macrocephalus still remains to be ascertained. 



The cervical region of the spine in this species exhibits the 

 prominent character of the genus in its great extension. It is, 

 however, only twice the length of the lower jaw, instead of three 

 times the length of the same part, as in the PL Hawkinsii ; and 

 this difference, arising from the greater development of the head 

 in the PL macrocephalus is associated, as Dr. Buckland has ob- 

 served,with thicker and stronger vertebrae in relation to the greater 

 weight they had to sustain. It includes twenty-nine cervical 

 vertebrae. In the twentieth cervical vertebra of PL HatuMnsii, 

 the transverse is to the antero-posterior diameter as 4 to 3. In 

 the corresponding vertebra of the PL rnacrocephalus the trans- 

 verse is to the antero-posterior diameter very nearly as 2 to 1. 

 The rest of the cervical vertebrae bear a similar ratio to those of 

 the PL Hawkinsii ', the bodies of the vertebrae therefore in PL 

 macrocephalus, although by no means so flat as in the Ich- 

 thyosauri, make an evident approach to the characteristic form 

 of the vertebrae in that genus. 



In the PL Hawkinsii the hatchet-shaped processes are con- 

 verted into stjdiform ribs at the twenty-ninth cervical vertebra ; 



