No. 408.] ORIGIN OF THE MAMMALIA. 945 
of the basioccipital ; other Chelonia 
are typically monocondylic. In like 
manner certain Anomodontia (Therio- 
dontia) are typically monocondylic ; 
others have a tripartite condyle as 
figured in Dicynodon (F); others 
again have a tripartite V-shaped con- 
dyle with the foramen magnum lying 
between the exoccipital portions ; 
finally, in Cynognathus (G), we have 
two prominent convex condyles on the 
lower sides of the foramen magnum 
composed chiefly of the exoccipitals. 
It is from the latter type, in our 
opinion, that the mammalian structure | 
was probably derived. 
But it is first necessary to demon- 
strate why the apparently obvious 
derivation of the mammalian from the 
amphibian condition is improbable. 
We do not know the origin of the 
condyles in the Amphibia; all we 
know is that as soon as the occipital 
region becomes osseous, the most 
primitive amphibians show paired 
exoccipital condyles with a median 
cartilaginous or reduced basioccipital 
element; the reduction of the basi- 
occipital is almost a class character. 
No Amphibia, so far as known to the 
writer, exhibit any participation of 
the basioccipital in the formation of 
the condyle, © 
On the contrary, many of the 
mammals show an extensive partici- 
pation of the basioccipital in the for- 
mation of the condyles ; in fact, some 
mammalian condylar types might 
D 
