324 CHELONIA 



more in the Trionychidae, the nuchal sends out a pair, of long 

 rib-like processes, which either extend to below some of the 

 neighbouring marginals, or their ends overlap those of the ribs 

 of the second thoracic vertebra (e.g. Trionyx), or, lastly, they 

 are in turn overlapped by the first costal plates (e.g. Gyclanorhis). 

 Such rib-like processes are also present, well developed in the 

 young, shorter in the adult, in the Dermatemydidae and Cino- 

 sternidae. It is possible that the nuchal plate represents the 

 fused neural of the eighth and the costal plates of the ninth 

 vertebrae. An indication of the compound nature of the nuchal 

 may be found in the fact that two nuchals have been described 

 in Chely drop sis carinata, a Miocene relation of Chelydra. Some- 

 what similar modifications have taken place in the post-sacral 

 region. The one to three supracaudal plates are, namely, 

 neurals which have lost their connexion with, or perhaps have 

 never been fused with, the spinous processes of the movable tail- 

 vertebrae. The number of neural plates is mostly eight, but 

 there are sometimes individually nine or ten, the gradual 

 suppression taking place first in the sacral region. When such 

 a plate is suppressed the neighbouring costal plates usually close 

 up and meet in the median line. In Cistudo, for instance, there 

 are only seven normal neurals, the eighth pair of costals meet, 

 and the original eighth neural is transformed into a supracaudal. 

 In Cinosternum the sixth to eighth costals meet, separating the 

 one supracaudal widely from the remaining five neurals. The 

 meeting of the last pair of costals, with co-ordinate reduction of 

 the neurals to seven, is almost universal in the Pleurodira ; and 

 this tendency is carried out to an extreme in the Brazilian 

 Platetnys and in the Australian Ghelodina and its allies, in 

 which aU the costals meet in the middle line, and the neurals 

 are completely suppressed. Every stage intermediate between 

 complete neurals {Sternothaerus) and interrupted, vestigial, and 

 vanished nem-als, is still represented by some genus. This pro- 

 cess takes place independently, both in America and in Australia, 

 and is one of the most recently introduced modifications. 



The costal plates arise, like the neurals, independently in the 

 cutis, but they soon come into contact with the underlying 

 cartilage of the ribs, which are long enough to reach the 

 marginals. The ribs flatten, become surrounded by the growing 

 membrane -bone of the plates, and the cartilage of the ribs. 



