CERVICAL VERTEBRAE OF CHELONIANS • 221 



fied with the ventral surface of the anterior margin of the first 

 neural plate of the carapace. Upon either side it extends back- 

 ward and outward as a slender rib that distinctly fuses, through 

 feeble coossification, with the anterior margin of the first true 

 dorsal rib just beyond its head. The forepart of this vertebra 

 no. 10 — its dorsal aspect — simply rests, mesially, against the 

 ventral aspect of the nuchal plate of the carapace, and at this 

 point it is articulated with vertebra no. 9, as shown in the accom- 

 panying figures on the plates. Our United States species of the 

 .soft-shelled turtles present identically the same arrangement 

 and morphology as this, and it is fair to presume that all other 

 true species of the genus Amyda do the same. 



Coming next to the snapping turtles (Chelydra), this vertebra 

 no. 10 has assumed a form that distinctly stamps it as a true 

 dorsal vertebra. It possesses a conspicuous neural spine that 

 barely reaches the articulation of the nuchal plate of the carapace, 

 anteriorly; it has a small pair of ribs of its own, either one of 

 which, for its outer two-thirds, fuses, in adult individuals, with 

 the anterior margin of the true dorsal rib of its own side, as far 

 as the middle point of the latter. Posteriorly, this vertebra 

 presents a demifacet upon either side of its centrum for articu- 

 lation with the anterior part of the head of the first true dorsal 

 rib next to it upon either hand. In old specimens of Chelydra 

 serpentina all of these osseous parts become thoroughly coossified 

 and form one solid piece. When this happens, the vertebra in 

 question is entirely and indistinguishably incorporated with the 

 animal's shell. Between Amyda and Chelydra, then, we note 

 a transitional stage, with respect to this tenth vertebra — a stage 

 where it is passing from a cervical to a typical dorsal one of the 

 series. While in all existing species known to me at this time, 

 the ribs are of a more or less rudimentary type, it is quite pos- 

 sible to conceive that in the progenitors of the line of chelonians, 

 from which the existing species of Amyda have arisen, this 

 particular vertebra was a true cervical one; that is to say, it 

 possessed no ribs and was in no way coossified with the carapace 

 of the shell in adult individuals. 



