INTRODUCTION. xv 



arrangement of all the elements are almost exactly as in Cryptocleidus. It should, 

 however, be noted that neither Eretmosaurus rugosus nox Microcleidus homalospondylus 

 fall within the limits of the family Elasmosauridse, because their cervical ribs are 

 double-headed, a condition not found in the true Elasmosaurs. 



The genus Elasmosaurus itself was founded by Cope * for the reception of several 

 species of Plesiosaurians from various Cretaceous deposits in Kansas and Nebraska. 

 The type species was described by Cope from an imperfect skeleton, consisting of the 

 greater part of the vertebral column and the imperfect limb-girdles, the pectoral 

 girdle showing the peculiar structure which has been regarded as of sufficient importance 

 to justify the establishment of a distinct family for the reception of the genera in 

 which it occurs. One peculiarity of Elasmosaurus proper is the enormous length of 

 the neck, which consists of no less than 76 vertebrae ; in Murcenosaurus, the English 

 genus in which the neck is longest, there are only 44 cervical vertebrae. 



Of the numerous other genera of American Plesiosaurs many are very imperfectly 

 known : some almost certainly belong to this family (e. g., Cimoliosaurus, in which 

 the shoulder-girdle is not yet known). In the genera such as Dolichorhjncops 

 and Brachauchenius, which have been described in detail by Williston f , the 

 shoulder-girdle is not Elasmosaurian ; these animals seem to approach rather the 

 Pliosaurian type, though they differ from the typical Pliosauridae in possessing 

 single-headed cervical ribs, and in some respects the structure of the skull seems 

 to be very different. 



In their habits the Plesiosaurs probably differed widely from the Ichthyosaurs. 

 In the first place, their mode of swimming was quite different, propulsion through 

 the water being effected entirely, or almost entirely, by the oar-like paddles, the 

 tail being short and, so far as is known, possessing no fin, or at most a very small 

 one. This manner of swimming and the great length of the neck are characters 

 preventing the supposition that these animals moved at a great pace beneath the 

 water, and it is much more likely that they lived mainly at the surface and 

 at no great distance from the shore. The shorter-necked Pliosaurs are more 



* Proc. Acad. Nat. Sci. Philadelphia, 18G8, p. 68. 



t "North-American Plesiosaurs, Pt. I.," Pield Columbian Museum — Greology, vol. ii. no. 1 (1903); "North- 

 American Plesiosaurs : Elasmosaurus, Cimoliosaurus, and Polycotylus," Amer. Journ. Sci. [4] vol. ssi. 

 (1906) p. 221; "The Skull of Brachauchenius, &c.," Proc. United States Nat. Mus. vol. sssii. (1907) 

 p. 477; " North- American Plesiosaurs: 'I'riiiacromerum," Journ. Geol. vol. xvi. (190S) p. 715. 



