944 THE AMERICAN NATURALIST. | [Vor. XXXIV. 
II. Monocondylic, Tripartite. 
Single condyle, basi- and exoccipitals in 
different proportions. 
Reptilia (e.g., Chelone, Fig. 1, B, C, D, £). 
III. Dicondylic, Transitional. 
Paired condyles, chiefly exoccipital, partly 
basioccipital. 
Reptilia (e.g., Sagi certain Mam- 
malia). 
IV. Dicondylic, Typical. 
Paired exoccipital condyles, basioccipital 
element cartilaginous or reduced in 
median line. 
Amphibia (Rana, Mastodonsaurus) and 
certain Mammalia. 
The above types are arranged morpho- 
logically, not genetically. Seeley (88, p. 
100) has pointed out the significance of 
the morphological transition from I to 
IV as consisting in the reduction of the 
basioccipital and ‘enlargement of the 
exoccipital elements ; he concludes: 
* Hence it is possible that the condylar 
cranial characters may be related in an 
evolutionary sequence of gradation, and 
that the change from one condition to 
another may be consequences of one 
plan of vertebrate organization rather 
than that of entirely independent plans." 
The diagrams (Fig. 1) have been pre- 
pared to illustrate the transition between 
these types. They show that in the alli- 
gator the condyle is wholly basioccipital 
and below the foramen magnum. In 
the lizard, python, and two genera of 
Chelonia we observe a gradual expan- 
sion of the exoccipitals and reduction 
Fi. 1 5 Mna condyles. A, Alligator lucius; B, I, a tuber tho 
Chelone midas; E, Testudo polyphemus; F, DE ynadon ake ETT g, e adito 
(her Seeley). A-D, Minnie, rapi typical al; EG G, reduction a a element. 
o., basioccipital ; Exo 5 
