528 ON A NEW SPECIES OF PLESIOSAURUS 



numbers in the one case would be 31+23 = 54, in the other 

 29 + 25 = 54- 



Or 3. Prof. Owen imagines that the total number of cervico-dorsal 

 vertebrae may vary between 52 (29 + 23) and 56 (31+25) in different 

 individuals of the same species. 



I am not aware that a shadow of evidence exists in favour of the 

 occurrence of so remarkable a variation as the last-named in any 

 vertebrate 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- 

 tebras, 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 z.ny P. Hawkinsii \x\ the British. 

 Museum. Under these circumstances it became necessary to inquire 

 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. i that — 



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

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

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

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



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

 vertebrae ; four times the same length equals the anterior 3 1 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 body. The proportions 

 of the head to the neck are as in No. i. 



In No. I 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 ;; 



