934 THE AMERICAN NATURALIST. [VoL. XXXII. 
origin in bone developed, not in, but beneath the skin ; and he 
suggested that the uncinate processes of the ribs of the croco- 
diles and birds might afford a clue to the solution of the prob- 
lem. However, these uncinate processes are of cartilaginous 
origin, while the bony plates which constitute the greater 
portion of the carapace are of membranous origin. 
We must recognize, at least in the Amniota, besides the 
bones developed from a basis of cartilage, two kinds of mem- 
brane bone. One of these is developed within the external 
integument itself ; the other in the fascia, beneath the skin. 
Examples of the first kind, or true dermal bones, may be found 
in the osteodermal plates which occur in the scales of the 
Scincidze and of some other lizards. Such, too, are the pieces 
which form the mosaic armor of Dermochelys. Examples of 
the second kind of bones, or fascia bones, are furnished by 
the so-called abdominal ribs of Sphenodon. Of course the two 
kinds may often coalesce with each other or with cartilage 
bones. 
In the abdomen of the cayman both strata of bones occur. 
There is a set of abdominal ribs developed in the subcutaneous 
fascia, while in the skin itself there is a system of bony scutes 
which constitute a ventral armor. Sphenodon likewise pos- 
sesses a system of abdominal ribs, which are wholly independ- 
ent of the true ribs ; but there is no dermal armor. If bony 
plates were developed in the ventral scales of Sphenodon, we 
would find two strata of bones, as in the cayman. 
The abdominal ribs of Sphenodon, then, are homologous with 
those of the cayman, and not with the latter’s dermal armor. 
It is generally agreed that most of the plastral bones of the 
turtles find their equivalents in the abdominal ribs of Sphe- 
nodon, not in the dermal armor of the cayman. The epiplastra 
and the entoplastron of the Testudines are doubtless the homo- 
logues of the clavicle and the interclavicle of other reptiles and 
of the Stegocephali, and belong to the same stratum of bone as 
the abdominal ribs. 
Now it seems to me almost certain that the marginal bones of 
turtles have had the same origin as the bones of the plastron ; 
that is, they are not dermal bones, but fascia bones. Further- 
