MODIFICATION IN TURTLES. 17 
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. ‘Lhe 
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 mehe latter show retardation in their dev elopment, 
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 Trionyx (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. 
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