CERVICAL VERTEBRAE OF CHELONIANS 219 



either persisting as a distinct os odontoideum, or anchylosing 

 with the body of the axis, and becoming the so-called odontoid 

 process of this vertebra" ('72, p. 18). 



We have here a declaration which it is evident sustains the 

 fact that in Amyda, and doubtless in other turtles, the so-called 

 odontoid bone is in reality the second cervical or axis vertebra, 

 inasmuch as we find it supports an odontoid process. 



Professor Huxley makes no mention, in the work cited, of the 

 fact that the postzygapophyses of the atlas extend backward to 

 articulate with the prezygapophyses of the third cervical verte- 

 bra, and in doing so pass over the axis vertebra as pointed out 

 above. There may be exceptions to this among the Chelonia, 

 but if so, the species are not recalled at this writing. The fact 

 that the axis has become largely vestigial in character is doubt- 

 less based in the manner of use of the neck in a turtle. The 

 rotary movements of the skull involving the axis are far more 

 limited than in other Vertebrata; wiiile the ability to draw the 

 head back into the carapace and to thrust it suddenly forward, 

 as most turtles do, demands a peculiar action in one plane — - 

 hence a certain abrogation of function in the distal end of the 

 cervical vertebrae; elongation of the median ones, and, again, 

 a shortening of those as the carapace is approached. We find 

 the same condition of things among such birds as bitterns, herons, 

 and other waders. 



All chelonians, irrespective of genus or family, in so far as I 

 have examined them, present a more or less sudden and pecuhar 

 change of form in the cervical vertebra marked .9 in these plates. 

 This is particularly true in the case of the soft-shelled forms of 

 the genus Amyda. Here, when viewed directly from above, it 

 is almost square in outline, being rather wider posteriorly than 

 it is in front. Both neural and haemal spines are entirely absent, 

 while the prezygapophyses are small, subcircular in outline, and 

 to some extent tilted backward. The extension backward of 

 their mesial margins are sharp, and by meeting in the middle 

 line, they form two sides of an equilateral triangle, the base of 

 which is an imaginary line joining the prezygapophyses ante- 

 riorly. Below this imaginary line in front, situated side by side, 



