ZOOLOGY. 231 
"Serial divergence from these lower and higher orders to an - 
extreme of special peculiarity, such as is mentioned in Prop. III. 
above, has been alluded to in the discussion of the homologies of 
the opisthotic and squamosal bones. This is seen in the successive 
prolongation of the elements on the sides of the posterior region 
of the cranium into a “suspensorium,” and the successive libera- 
tion of the quadrate bone from several sutural articulations, to a 
condition as a mobile fulcrum for the mandible. This succession is 
seen first in the Rhynchocephalia, where the suspensorium is pro- 
duced, but the quadrate fixed; the Testudinata, where the quad- 
rate is freed from a quadratojugal bone; in the Lacertilia, where . - 
the quadrate is movable, but the opisthotic not produced; in the 
Pythonomorpha, where the opisthotic is produced as suspensorium ; 
the extreme being reached in the Ophidia, where the suspensorium 
itself becomes movable, and with it the elements which usually 
form the solid surface of the palate. 
This series then, it is evident, is like that of the Teleostei, among 
the lower Vertebrata, a special divergence from the main line of 
succession to the higher classes. The reptiles which retain and 
increase the close contact of the quadrate bone with the periotic 
elements are evidently those which conduct us to the Mammalia. 
The. highest group in this succession is the Crocodilia. Those 
which consolidate the periotic elements, but retain the partial free- 
dom of the quadrate, on the other hand, lead to the Avine class. 
These are the Ornithosauria, and perhaps, when we come to know 
the cranium, the Dinosauria. At least this may be predicated, if 
the structure of the foot and ear bones are correlated in this group 
as.they are elsewhere. 
The primary importance of this series is confirmed by the corre- 
lation with it of the serial modification of the modes of attachment 
of the ribs. These differences were first used in systematic work 
by Owen,*. and later more fully by Huxley.t The latter sub- 
divides the Reptilia in accordance with it alone, and, while pointing 
out important affinities thereby, fails to recognize others from his 
neglect of the modifications of the quadrate and supporting 
bones. 
In the most generalized form (represented by Ichthyosaurus), 
the capitular and tubercular articular surfaces are near together, 
but distinct, and situate on the sides of the vertebral centra. 
* Paleontology. + Jour. Geol. Society, London, 1870. 
