ROOM III. PLESIOSAURUS ARCUATUS. 349 



Owen, in his masterly review of the osteology of the Enalio- 

 sauriaus, has established its specific distinction, and assigned 

 to it the name of its discoverer. There are three other re- 

 markably choice examples of this species in Cases E and F. 

 That in the latter case is the most exquisite fossil skeleton in 

 the British Museum ; the perfection of the bones, the ad- 

 mirable manner in which the stone has been chiselled away, 

 and the graceful position of the neck, head, and limbs, render 

 this precious relic invaluable. 



The specific characters which distinguish the Plesiosaurus 

 Hawkinsii from the typical P. dolichodeirus, are chiefly dif- 

 ferences in the relative proportion of various parts of the 

 skeleton. 



The head is somewhat larger : it is three times the length 

 of the neck, instead of being four times, as in P. dolicho- 

 deirus. The length of the neck only slightly exceeds that of 

 the trunk, whereas in the latter it is equal to that of the 

 united body and tail. The number of cervical vertebrae is 

 twenty-nine ; in P. dolichodeirus, thirty-five. 1 There' are also 

 recognisable differences in the forms and relative sizes of the 

 ulna and tibia. 



PLESIOSAURUS ARCUATUS. Wall-case D : and on the top of 

 Wall-case E. Many detached bones of this species are placed 

 on each side the specimens above described in Case D. 

 This species is distinguished by the development of distinct 

 transverse processes, from the sides of the centrum of the ver- 

 tebra, for the support of the cervical ribs, especially from 

 those of the posterior moiety of the cervical region. " These 

 processes have the articular surfaces traversed by a longitu- 

 dinal groove, as in other Plesiosauri, and, consequently, thus 

 present the appearance of the two normal transverse processes 



1 Cei-vical vertebra. Professor Owen reckons as cervicals those in 

 which the centrum or body of the vertebra bears the whole, or a part of 

 the costal articular surface. " The body of a cervical may always be dis- 

 tinguished from that of a caudal vertebra in being without any trace of 

 haemapophysial pits. The dorsal vertebrae are those in which the 

 costal surface is situated wholly on the neurapophysis. The caudal 

 vertebrae are characterised by having both costal and neurapophysial 

 impressions on the body, except the terminal ones, which are readily 

 distinguished by their small size, the absence of both the above-named 

 impressions, and by the concave character of their articular surfaces." 

 ' Brit. Assoc. Reports," 1839, p. 58. 



