286 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Jan. 6, 



tebrate animal so highly organized as the Plesiosaurus. If the 

 second be the right interpretation of Prof. Owen's views, then P. 

 Hawkinsii will always have the same number of cervico-dorsal ver- 

 tebrae, and that number is according to Prof. Owen at least fifty-four, 

 and at most fifty-six. I have shown, however, that in P. Etheridgii 

 there are only fifty-three cervico-dorsal vertebrae. 



I beg to repeat, however, that I can find no proof of the existence 

 of fifty-four cervico-dorsal vertebrae in any P. Hawkinsii in the Bri- 

 tish Museum. Under these circumstances it became necessary to in- 

 quire whether the proportions of the head, body, and neck might not 

 furnish the needful marks of specific distinction. Measuring these 

 in the same way as P. Etheridgii, I find with regard to No. 1 that — 



1 . Taking the length of the skull (from the occipital condyle to 

 the end of the snout) as 1, the whole body measures between 10 and 

 1 1 . Taking the head from the end of the snout to the end of the 

 lower jaw as 1, the whole body measures between 8 and 9. 



2. Three times the length of the skull equals the anterior 25 ver- 

 tebrae ; four times the same length equals the anterior 31 vertebrae. 



In No. 2 the end of the tail is gone, and therefore the proportions 

 of the skull to the entire body cannot be ascertained ; but three times 

 the length of the skull measured along the neck reaches the middle 

 of the twenty-fifth cervical vertebra, and four times equals the 31 

 vertebrae as before. 



In No. 3 the length of the skull is rather less than one-eleventh 

 of the whole body ; while the length from the snout to the angle of 

 the jaw is rather less than one-ninth of the whole hodj. The propor- 

 tions of the head to the neck are as in No. 1 . 



In No. 1 the rib of the 29th cervical vertebra is hatchet-shaped ; 

 the shape of the ribs of the 30th and 31st vertebrae is not certainly 

 discoverable, nor can the character of their articular surfaces be clearly 

 made out. 



In No. 3 the rib of the 29th vertebra is truly hatchet-shaped. 

 Those of the 31st vertebra cannot be made out clearly, nor can that 

 of the 30th on the left-hand side. On the right side the head of a 

 rib lies against the posterior part of the neural arch of this vertebra ; 

 and, though its produced angle is more or less broken, its hatchet- 

 shape can be clearly distinguished. The costal articular facets of 

 both the 30th and 31st vertebrae are traversed by the neurapophy- 

 sial suture. 



In No. 2 the condition of the posterior cervical vertebrae is such 

 as to render it very unsafe to speak decidedly as to the character of 

 either the ribs or their articular facets. 



So far as these specimens go, then, they favour the idea that P. 

 Hawkinsii has 31 vertebrae cervical in Prof. Owen's sense of the 

 term, and they assuredly do not countenance the notion that these 

 vertebrae may vary in the same species. But if Plesiosaurus Haw- 

 kinsii has 54 or 56 cervico-dorsal vertebrae, and if 31 of these are cer- 

 vical, then P. Etheridgii differs from it in the following particulars : — 



1 . The number of cervical vertebrae is at least one less. 



2. The number of cervico-dorsal vertebrae is one or three less. 



