6 Eobert E. Coker. 



a comparatively small number of miits in eacli series. This armoi 

 overlapped, or ''broke joints," with the yet imperfectly developed 

 cartilaginous carapace, but with the further development of the latter, 

 and the consequent loss of the usefulness of the outer armor to the 

 animal, the dermal skeleton became reduced and finally disappeared. 

 The epidermal scutes, corresponding to these plates, remain, how- 

 ever, and from their present arrangement in the carapace of The- 

 cophorous turtles we are enabled to infer something of the modifica- 

 tions which the dermal armor underwent. Probably the last rem- 

 nants of the dermal plates persisted as small ossicles at the keel 

 prominences that are noticeable in adults of some species, but espe- 

 cially in the young of certain species (e. g., of Thalassochelys, 

 cf. Fig. 74:). In the further course of evolution these remnants 

 became either completely reduced or merged into the deeper plates 

 underlying them; but Hay has observed at least one, and probably 

 two such ossicles in the neural series of a fossil specimen of 

 Toxochelys. 



Thus, in Hay's view, the series of scutes of the carapace are 

 directly homologous to the series of epidermal areas overlying the 

 plates of the keels of a young Dermochelys. The neural series 

 corresponds to the median dorsal keel. Of the two lateral dorsal 

 keels, one is represented by the costals, while the other is lost in 

 most turtles, but preserved as a short series of supra-marginals in 

 Macroclemmys. Marginal keels correspond to marginal scutes. Of 

 the two lateral ventral keels, the internal gave rise to the plastral 

 scutes, while the external is more or less reduced, but still survives 

 in most turtles, either as a continuous series of inframarginals (sea 

 turtles, etc.), or as isolated axillary and inguinal shields. In some 

 land tortoises this series is entirely unrepresented. The median 

 ventral is almost entirely lost, but remnants are seen, for example, 

 in the characteristic unpaired intergular of Chelodina, and in the 

 occasional occurrence in other species of small unpaired scutes in 

 the median line of the plastron. Such a scute is most commonly 

 found just at the apices of the gulars and is referred to as an 

 intergular," or, better, as an "interplastral." 

 An hypothesis advocated by J^ewmann follows in logical order 



i(' 



