262 KANSAS UNIVERSITY SCIENCE BULLETIN. 



though modern text-books still continue to affirm their absence 

 in these reptiles. They doubtless furnished better protection 

 for the eyes under varying pressure, and perhaps aided vision, 

 both of which would indicate that these animals were in the 

 habit of going to considerable depths. 



The shortening of the neck is carried to the extreme in the 

 ichthyosaurs and cetaceans, the most specialized of aquatic air- 

 breathing vertebrates. In the mosasaurs there has been a ma- 

 terial reduction in the length of the neck and in the number of 

 the vertebrae from the ancestral Dolichosaur-like squamate. A 

 similar reduction in length and in number (seven) is also found 

 in the thalattosuchian crocodiles, according to Fraas. Among 

 the plesiosaurs there is extraordinary variability in the length 

 of the neck and in the number of the vertebrae. In such forms 

 as Elasmosaurus, from the Pierre Cretaceous, as many as forty 

 vertebrae are credited to the cervical region, and the most of 

 them of an elongated shape. In Dolichorhynchops, on the other 

 hand, there are not more than twenty vertebrae in the same 

 region, and all are very short. Both types may have been 

 contemporaneous in the latter time of the plesiosaur's existence. 

 How far the correlation of short neck and elongated skull ob- 

 tains in this order I do not know, but I believe that such is the 

 rule. I can conceive of no terrestrial form with so extraordinary 

 a neck as have some of the plesiosaurs, and the conclusion is 

 unavoidable that there has been an actual increase in the num- 

 ber, even as it is certain that there has been an increase in the 

 number of vertebrae in the more modern snakes. It is also evi- 

 dent that there has been a decrease in number from the long- 

 necked to the short-necked plesiosaurs. Perhaps in no other 

 order of vertebrates has there been such great variability in the 

 number of cervical vertebrae as in the plesiosaurs. 



The elongation of the trunk is constant in the more special- 

 ized aquatic types, and there is in such a progressive weakness 

 or obsolescence of the zygapophysial articulations posteriorly, 

 giving greater freedom of movement laterally. The elongation 

 of the tail and the development of a distal fin are correlated 

 with the weakness or absence of the hind limbs, as fins or pad- 

 dles. In the mammals the fin is placed horizontally, more for 

 the purpose of deep-sea diving than for speed or quickness of 

 evolution, which are needed in but few of these animals. In 



