1885. ] Frogressive and Retrogressive. 343 
still more unspecialized feet and limbs of the Ichthyosaurus (Ich- 
thyopterygia) cannot yet be ascribed to degeneracy, for their his- 
tory is too little known. At the end of the line the snakes pre- 
sent us with another evidence of degeneracy. But few have a 
pelvic arch (Stenostomide Peters), while very few (Peropoda) 
have any trace of a posterior limb. 
The vertebrz are not introduced into the definitions of the 
orders, since they are not so exclusively distinctive as many other 
parts of the skeleton. They nevertheless must not be over- 
looked. As in the Batrachia the Permian orders show infe- 
riority in the deficient ossification of the centrum. Many of the 
Theromorpha are notochordal, a character not found in any later 
order of reptiles excepting ina few Lacertilia (Gecconidz), They 
thus differ from the Mammalia, whose characters are approached 
more nearly by some of the terrestrial Dinosauria in this respect. 
Leaving this order we soon reach the prevalent ball and socket 
type of the majority of Reptilia. This strong kind of articula- 
tion is a need which accompanies the more elongated column 
which itself results at first from the posterior direction of the 
ilium. In the order with the longest column, the Ophidia, a sec- 
ond articulation, the zygosphen, is introduced. The mechanical 
value of the later reptilian vertebral structure is obvious, and in 
this respect the class may be said to present a higher or more 
perfect condition than the Mammalia. 
In review it may be said of the reptilian line, that it exhibits 
marked degeneracy in its’ skeletal structure since the Permian 
epoch ; the exception to this statement being in the nature of the 
articulations of the vertebrze. And this specialization is an adap- 
-tation to one of the conditions of degeneracy, viz., the weakening 
and final loss of the limbs and the arches to which they are at- 
tached. r 
The history of the development of the brain in the Reptilia 
presents some interesting facts. In the Diadectid family of the 
Permian Theromorpha it is smaller than in a Boa constrictor, but 
larger than in some of the Jurassic Dinosauria. Marsh has 
shown that some of the latter possess brains of relatively very 
narrow hemispheres, so that in this organ those gigantic reptiles 
were degenerate, while the existing streptostylicate orders have 
advanced beyond their Permian ancestors. 
There are many remarkable cases of what may now be safely 
