74 H. H. NEWMAN. 



neural spines have even commenced to broaden out and that the 

 procaudals and the marginals follow before the neurals and costals 

 are completely organized, points to the antiquity of these dermal 

 structures and indicates that the neurals and costals are of more 

 recent origin. 



3. Comparative anatomy furnishes us much valuable evidence. 

 In the family Trionychidae, for example, we have a series of 

 forms that show a gradual reduction of a portion of the dermal 

 armor. Fossil Trionychidae are well known in which are shown 

 a nuchal, a procaudal and a nearly complete set of marginal 

 plates. Such a form was figured by Dollo in 1884 and named 

 by him Pseudotrionyx. Another fossil species discovered by the 

 same palaeontologist and named by him Emyda granosa, lacks 

 the procaudal and the marginals from the anterior half of the 

 carapace. A third form, Emyda Ceylonemis, possesses a nuchal 

 and several marginals at the posterior part of the carapace. The 

 extreme limit of reduction is seen in Aspidonectes spiuifcr, which 

 possesses only the nuchal plate as the last remnant of the dermal 

 carapace. 



It will be noted that the order of the appearance of these 

 dermal ossifications in ontogeny is just the inverse of the order 

 of disappearance in phylogeny. The latest elements to be 

 formed in ontogeny are the first to disappear in phylogeny. This 

 is just what we would expect if we consider that there has been 

 a gradual shortening of the developmental process, a gradual 

 elimination of the latest stages. The Trionychidae show clearly 

 that there is a marked tendency to reduce the system of dermal 

 bones and it is not difficult to imagine that earlier reduction has 

 taken place in which the dermal ossifications of midneural and 

 costal regions were lost. 



What evidence have we that such dermal ossifications over- 

 lying neural processes and ribs actually existed ? O. P. Hay 

 ('97) in an important paper dealing with the evolution of the 

 chelonian carapace and plastron, describes and pictures an in- 

 complete carapace of a fossil form named Toxochelys serrifer. 

 Three ossicles occur above and overlapping the neural plates and 

 occupy positions coincident with the keels of the second, third 

 and fourth neural scutes. These ossicles have the general form 



