76 H. H. NEWMAN. 



tures, considering the scutes as of little significance. Hay's view 

 of the role of the scutes may be stated briefly as follows : The 

 probable ancestral condition is that seen in Dermachelys, the skin 

 of which is found to be broken up into small polygonal areas, 

 larger in the keels than elsewhere. These areas coincided with 

 the osteodermal plates that are or will be developed in the skin. 

 As the deeper elements of the carapace (neural and costal plates) 

 increased in protective efficiency, the dermal structures were in 

 many regions rendered superfluous and disappeared. In some 

 cases the scutes were lost with their corresponding plates, in 

 others the lost plate left its trace in the keel of the scute. The 

 direction of growth of each of the existing series of scutes shows 

 the direction of encroachment on other rows now lost. 



This exposition of Hay's seems to me to be the most rational 

 yet advanced, yet I believe that he fails to appreciate the evidence 

 of embryology and thus introduces undue complexity. In the 

 first place, he considers the nuchal plate as a fascia bone instead 

 of an ordinary dermal plate. In the second place he states that 

 the neural and costal plates are of the same character as the 

 nuchal. Embryology shows that the nuchal plate is as true a 

 dermal bone as are the marginals, while the neurals and costals 

 are true periosteal expansions. It seems to me more rational to 

 suppose that the dermal ossifications of the mid-neural and costal 

 regions have undergone a complete suppression identical with 

 that indicated by the series of Trionychidae described above, rather 

 than that they have become indistinguishable by fusion with the 

 rib and neural fascia bones, as Hay calls them. 



If we remove the scutes and underlying dermis from the cara- 

 pace of a specimen of CJielydra we find that the long tubercles on 

 the neural and costal plates bear no constant relation to the 

 plates themselves, but are nevertheless clearly of a piece with 

 them. It was natural for Hay to suppose that these bony 

 tubercles were produced separately and then fused with the 

 underlying plates. I have been able to trace this matter to a 

 conclusion in the young of Chclydra, with the result that I have 

 seen all the stages of ossification in the carapace and know that 

 the tubercular keels on the neural and costal plates are produced 

 by gradual thickenings of the growing plates. These thickenings 



