MODIFICATION IN TURTLES. 1 7 



ON THE AMOUNT OF MODIFICATION UNDERGONE BY TURTLES 

 SINCE THEIR EARLIEST APPEARANCE. 



On preceding pages a brief exposition has been presented of the principal 

 structures shown in the skeletons of turtles of various groups. It is now intended 

 to consider the differences of structure from another point of view, that of determin- 

 ing the amount of differentiation these animals have suffered since their earliest 

 appearance, especially as compared with the changes undergone by other orders of 

 reptiles. It is generally supposed that turtles have been unusually conservative in 

 their changes; that, as in their movements so in their development, they have 

 progrest slowly. We are to inquire to what extent this impression is true. 



Let us first consider the shell, that part which is most often found preserved in 

 the rocks and the part which is regarded as most characteristic of turtles. The 

 theory accepted by the writer is that originally the upper side of the body was 

 protected by a rim of peripheral bones, a median row of neural bones, and eight 

 pairs of bony plates which overlay the ribs and had possibly in the earliest turtle 

 coalesct with them, probably joining one another by their contiguous edges; and 

 that in addition to the bones enumerated, subdermal in their position, there was a 

 more superficial layer of bones, dermal in position and forming seven longitudinal 

 zones, a median or vertebral zone, two costal, two supramarginal, and two marginal 

 zones. The plastron was formed of at least eleven subdermal bones, while super- 

 ficially there were found five zones of dermal bones, a median and two lateral on 

 each side of it. Starting with this outfit, the vast majority of turtles have wholly 

 or almost wholly divested themselves of the dermal layers and have acquired a solid 

 shell composed of the subdermal bones. On the other hand, the leatherback 

 turtle, Dermochelys, appears to have retained the dermal and to have almost wholly 

 surrendered the subdermal bones, for the costal plates now form only unimportant 

 fringes on the ribs; the peripherals and neurals are wholly gone; the nuchal 

 is reduced; and the plastral bones are only eight slender rods. As regards the 

 carapace, Dermochelys possesses little that is homologous with that of most other 

 turtles. 



If we view the modifications undergone by the shell within the group known as 

 Thecophora we find that they are extensive. We can hardly doubt that the primi- 

 tive turtles possest a pair of mesoplastral bones; and yet we know that all turtles 

 have discarded these, except a few of the Pleurodira. Within the latter super- 

 family the shell has not suffered remarkable modifications. Nevertheless, most 

 of the genera have no mesoplastra. In some the plastron is connected with the 

 carapace ligamentously, while in others strong axillary and inguinal buttresses rise 

 from the plastron and articulate with the inferior surface of some of the costals. In 

 all, the hindermost costals have contracted a sutural union with the ilia. In a 

 considerable number of genera the neurals have become wholly supprest. In 

 Pelusios (Sternotharus) there is a hinge behind the hyoplastra. 



In the superfamily Trionychoidea the shell has undergone extensive reduction. 

 No traces are known of the original dermal layer of bones. The neurals and the 

 costal plates are retained; but the latter show retardation in their development, 

 attaining the distal ends of the ribs not at all or only at a late date in life. The 

 peripherals are wholly missing, except perhaps in the genus Tnonyx (Emyda Gray). 

 In the plastron we never find mesoplastra and all the bones are more or less reduced. 

 In the great majority of cases there are fontanels on the midline and at the bridges. 

 The form and relations of the entoplastron and the epiplastra are very different 

 from those of other turtles, but homologies are easily traced. 



