36 Field Columrian Museum — Geology, Vol. II. 



ments in the reptilia is found in Ichthyosaurus, in which, as shown 

 in the figures given of the axis and atlas by Owen, the intercentra 

 of the first three vertebrae occupy nearly their normal and unaltered 

 relations to the vertebrae. It is difficult to understand, however, how 

 such a primitive condition of these parts could have been inherited 

 from a terrestrial ancestor. In the plesiosaurs the specialization has 

 been carried further, still the structure is yet more primitive than is 

 known in any modern reptiles and in most of the extinct. We cerx 

 tainly can look for ancestral forms of these vertebrae among the 

 Stegocephalia or Anomodonts only; in all other reptilia they 

 have acquired too great specialization to easily revert to the gen- 

 eralized structure. 



In the modern crocodiles the specialization has been carried so 

 far that the axial rib has become supported by the atlantal centrum 

 only, while the atlantal rib has been pushed forward on the atlantal 

 intercentrum. 



It is of interest to observe that in Shastosaun/s of the earlier Ich- 

 thyopterygia, according to Merriam, there were probably five inter- 

 centra present in the anterior cervical region, while in Ichthyosaurus 

 there are but three, and in the more specialized forms, Baptanodon* 

 they have entirely disappeared. 



Beyond the axis there are seventeen distinctly cervical 7'crtchnc 

 preserved, together with one or two transitional ones, which must be 

 classed with the dorsals, however. I believe that these were all that 

 the animal possessed, though it is possible there may have been one 

 or two more. The arches and ribs of all, save of the third, were 

 detached and scattered about among the other bones, so that much 

 difficulty was encountered in properly associating the parts. Because 

 of a gradual increase in size of the ribs and arches, as well as the 

 centra, it would seem that the final collocation made is correct, and 

 because all these processes agree in number it would seem still more 

 probable that no vertebrae are missing. (See Pis. VI-IX.) 



The centra increase in height and width gradually throughout the 

 series. The third has a transverse diameter of thirty-five millimeters, 

 a height of thirty and length of twenty-eight. The sixteenth centrum 

 has its corresponding diameters as follows: forty-five, thirty-eight and 

 twenty-six. The three following are somewhat distorted, but seem 

 likewise to increase slightly in height and width. It will be observed 

 that the length is nearly or quite the same in all, the differences being 

 exhibited in the width and height only. The articular surfaces of the 

 centra are gently and evenly concave, with a slight rounded eminence 



*W. C. Knight, in Hit. 



