24 FOSSIL TURTLES OF NORTH AMERICA. 



originally was, but it is now overlain by the dermal mosaic. Mr. Newman holds also 

 that the peripherals and the suprapygals are dermal bones, not what the writer called 

 fascia bones or subdermal bones. If the reader will consult the succeeding pages that 

 deal with Toxochelys he will see that one of the dermal nodules overlies the second 

 suprapygal. T he latter is therefore not equivalent to the row of nodules in front 

 of it and to those on the tail of the snapping-turtle, but is a bone of a deeper layer. 



We may, therefore, for the present regard the characters of the most primitive 

 turtles as having been the following: There was a superficial armor composed of 

 dermal bones arranged in at least 7 zones above and at least 5 zones below, with 

 many bones in each zone, and each bone covered by a horny scute. Beneath this 

 was a subdermal armor. On the upper side of the body, this consisted of the nuchal, 

 a row of neurals, 8 pairs of costal plates, suprapygals, and peripherals; below, of 

 the entoplastron and at least 5 pairs of subdermal bones. There were 18 presacral 

 vertebrae, of which 10 belonged to the trunk and were without transverse processes 

 and more or less immovably joined to each other and to the ribs and neural plates. 

 The neck was short and consisted of 8 biconcave vertebrae, which possest transverse 

 processes and low neural spines. There may possibly have been present some 

 cervical ribs. There were 2 sacrals. The tail consisted of biconcave vertebrae, 

 each provided with free ribs, with chevron bones below, and perhaps with one or 

 more rows of bony nodules above. 



The skull had a complete temporal roof, but the temporal fossa was probably 

 not widely open behind. There were nasals, lacrimals, and prefrontals, all 

 distinct from one another. There was no parietal foramen. A vomer was present 

 but no prevomers. The choanae opened far forward in the roof of the mouth. The 

 quadrate was fixt, somewhat excavated to form a tympanic cavity and notcht 

 behind for the stapes. Probably the palate was closed by the union of the pterygoids 

 with each other and with the basisphenoid. The parietals may or may not have 

 sent down each a plate to the pterygoid, in front of the exit of the trigeminal nerve. 

 The paroccipitals were free from the exoccipitals. The jaws were without teeth and 

 were covered by a sheath of horn. Each ramus of the lower jaw was doubtless com- 

 posed of 7 elements, and the dentaries were probably not co-ossified at symphysis. The 

 outer surface of the bones of the skull was covered by horny scutes. The shoulder- 

 girdle consisted of a pair of coracoids, a pair of scapulae, and a pair of procoracoids; 

 the latter probably co-ossifying somewhat late in life with the scapula. Limbs, fore 

 and hinder, probably not greatly different from those of living Chelydridae, but more 

 crudely modeled. The phalangeal formula was 2, 3, 3, 3, 3 in all the feet. 



If the views exprest above regarding the original composition of the armor of 

 turtles is correct, the possession of a carapace consisting exclusively of a mosaic 

 of dermal plates, as in Dermochelys, is a secondary character; as is likewise the 

 possession of a shell constituted only of the deeper elements, as in the great majority 

 of turtles. Other secondary characters are the absence of entoplastron {Dermo- 

 chelys, Kinosternon), the absence of peripherals (Trionychoidea), and the absence of 

 mesoplastra, as in most living turtles. Such too is the lack of nasals and lacri- 

 mals in the great majority of turtles. The adaptation of the limbs for habitual 

 swimming is, of course, a secondary modification of these organs. 



In this category, too, belong those modifications of the cervical vertebrae by 

 virtue of which the neck has become so greatly elongated in most forms and made 

 most freely flexible in a horizontal plane in the Pleurodira and in a perpendicular 

 plane in all the others. Most extraordinary of all the secondary characters is that 

 complex of modifications which permits the head and neck to be retracted between 

 the scapulae, the loop in the neck sometimes reaching backward quite to the pelvis. 



