160 MARINE SAURIANS. 



Neck. 



The most anomalous of all the characters of P. Dolicho- 

 deirus is the extraordinary extension of the neck, to a length 

 almost equalling that of the body and tail together, and sur- 

 })assing in the number of its vertebree (about thirty-three) 

 that of the most long-necked bird, the Swan : it thus de- 

 viates in the greatest degree from the almost universal law, 

 which limits the cervical vertebra; of quadrupeds to a very 

 small number. Even in the Camelopard, the Camel, and 

 Lama, their number is uniformly seven. In the short neck 

 of the Cetacea the type of this number is maintained. In 

 Birds it varies from nine to twenty-three ; and in living 

 Reptiles from three to eight.* We shall presently find in 



sumcs, in the renovation of its teeth, the characterof Lizards, combined with 

 the position of the perfect teeth in distinct alveoli, after the manner of Cro- 

 codiles. 



The number of teeth in the lower jaw was fifty-four, which, if met by a 

 corresponding series in the upper jaw, nmst have made the total number to 

 exceed one hundred. The anterior part of the extremity of the jaw enlarges 

 itself like the bowl of a spoon, to allow space for the reception of the first 

 six teeth on each side, which are the largest of all. 



* To compensate for the weakness that would have attended this great 

 elongation of the neck, the Plesiosaurus had an addition of a series of hatchet- 

 shaped processes, on each side of the lower part of the cervical vertebra?. 

 (PI. 17, and PI. 19, I, 2.) Rudiments and modifications of these processes 

 exist in birds, and in long-nceked quadrupeds. In the Crocodiles they 

 assume a form, most nearly approaching that which they bear in the Plesio- 

 saurus. 



The bodies of the vertebree also more nearly resemble tliose of certain 

 fossil Crocodiles, than of Ichthyosauri or Lizards; they agree farther vyith 

 the Crocodile, in having the annular part attached to the body by sutures; 

 so that we have in the neck of the P. Dolichodeirus a principle of con- 

 struction resembling that of the vertebrm of Crocodiles; combined with an 

 elongation very much exceeding that of the longest neck in birds, and such 

 as occurs in no other known animal of the extinct or living creations. 

 The length of the neck in P. Dolichodeirus is nearly five times that of 

 the head ; that of the trunk four times the length of the head, and of the 

 tail three times ; the head itself being one-thirteenth part of the whole 

 body.— See Geol. Trans. Lond. Vol. 5, p. 559, and Vol. I. N. S. p. 103, et 

 seq. 



