? 
36 FIELD COLUMBIAN MusruM—GEoOLoGy, Volt. II. 
ments in the reptilia is found in /chthyosaurus, 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 vertebra. 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 cer- 
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. 7 
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 Shastosaurus of the earlier Ich- 
thyopterygia, according to Merriam, there were probably five inter- 
centra present in the anterior cervical region, while in /chthyosaurus 
there are but three, and in the more specialized forms, Baptanodon,* 
they have entirely disappeared. 
Beyond the axis there are seventeen distinctly cerzvzcal vertebra 
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, 1t 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 vertebre are missing. (See Pls. 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, 77 /777. 
