84 S. W. Williston. 
It is generally assumed that the primitive vertebra from which 
those of higher types have been developed was composed of several 
distinct and separated bones, the neurocentra, pleurocentra, and 
hypocentra, and perhaps others. It must also be assumed that the 
differentiation of the three or four anterior vertebre to form the 
brain capsule took place before the elimination of those parts now 
absent in the higher animals. We should then expect to find in the 
vertebrate skull the separate elements or some of them at least, more 
or less differentiated in the embryonic condition. Furthermore we 
know that, originally, the ribs were continued as vertebral append- 
ages as far forward as the skull itself, as is evidenced by the atlantal 
ribs still existent in the crocodiles, pelycosaurs, and other reptiles. 
And why may we not also assume that the ribs originally continued 
on the cranial vertebre 4? The brain capsule of Labidosaurus yet pre- 
sents so much of its original vertebral condition that it has not 
become attached to the membrane bones of the cranial walls, and its 
occipital segment seems to present all the features of an undiffer- 
entiated vertebra, the basioecipital representing the hypocentrum, 
the exoccipitals the pleurocentra, the supraoccipital the neurocentra, 
while the paroccipitals have all the characters of real ribs. To 
carry the homologies to the next vertebra we would find the basi- 
sphenoid homologous with the hypocentrum, the prodtics with the 
pleurocentra, the alisphenoids or epipterygoids with the neurocentra, 
while the stapes occupies the ideal position of the primitive rib. That 
the stapes of the reptiles is not identical with that of the mammals 
is, I believe, usually conceded; that it did primarily originate as 
the rib seems not at all improbable. In this connection it is worth 
while to observe that the stapes of the primitive reptiles seem always 
of large size, a character also found in both the ichthyosaurs and 
plesiosaurs. 
