No. 384.] CHELONIAN CARAPACE AND PLASTRON. 935 
more, I see no reason why we may not regard the nuchal bone 
and those plates of bone which have united with the neural 
spines to form the neuralia, and with the ribs to form the 
costalia, as having originated in the same way. Two strata of 
bones might as reasonably be expected to occur on the dorsal 
region of the body as on the ventral. In accounting for the 
condition of the carapaces of modern turtles, we may suppose 
that the earliest ancestors of turtles had a scaly skin, which 
contained osteodermal plates.! Beneath these there were devel- 
oped first, perhaps, in the fascia of the shoulders, a nuchal bone, 
later other plates which in time became transformed into the 
neuralia and costalia. As these deeper-seated fascia bones 
increased in importance, the osteodermal plates underwent grad- 
ual reduction. Only in Dermochelys have they maintained 
anything like their early importance. As regards the deeper 
layer of bones even in this turtle, the ribs, flattened, and with 
jagged edges, seem to me to indicate that at some time in the 
remote past there have been costal plates of membrane bone 
fused with them. 
Can we find any evidences bearing on the hypothesis pro- 
posed ? 
In Vol. iv of the University Geological Survey of Kansas, 
pp. 370 et seq., Case has described and given figures of three 
species of the genus Toxochelys, not uncommon turtles of the 
upper Cretaceous deposits of Kansas. While working in Dr. 
Baur’s laboratory in the University of Chicago, I had the privi- 
lege of studying and of making drawings of the specimen of 
T. serrifer, which Case has presented on his Pl. LXXXIII. 
This specimen is the property of the ee depart- 
ment of the University of Kan- 
sas, now in charge of Dr. S. W. 
Williston. One of the most in- 
teresting observations that re- 
sulted, one that has often been 
recalled to mind, was that there 
was evidently a series of separate bones along the middle of the 
back, lying across certain of the articulations between neurals. 
1 Baur, G. Biol. Centralblatt, Bd. ix, p. 182; Sci., vol. xi, p. 144. 
