204 B. NATURAL HISTORY. 
which it seems to be continuous, as I cannot see any suture sepa- 
rating them. 
The basis cranii is incomplete, and is formed of basioccipital 
and basisphenoid. The latter (Cuvier, Oss. Foss. Tab. 257, figs. 
12, 13) supports an alisphenoid on each side. 
In considering the affinities of Ichthyosaurus as exhibited by the 
cranium, it may be premised that the structure of the limbs sepa- 
rates it as a very distinct order among Reptilia. The peculiar 
disposition of the squamosal is only paralleled among Anomodontia 
and Rhynchocephalia, and the character of the columella resem- 
bles only that of the former in its connections. The occipital 
elements have more the disposition of those of Sphenodon than of 
any other type, but there is a great difference in the position of 
the opisthotic. The arches are also those of the same genus, ex- 
cept that in the latter the quadratojugal is obsolete, or codssified 
with the malar. The structure of the front and base of the skull, 
and of the mandible, in Sphenodon, have no resemblance to those 
of Ichthyosaurus. The anteriorly unossified brain-case is that of 
several other Reptilian groups, while the presence of the alisphe- 
noid furnishes a point of resemblance to the Crocodilia. 
In general there are few points of affinity to the Crocodilia. The 
characters of the parietal bone are those of Sphenodon. The ver- 
tebre are intermediate between those of that genus and the Lacer- 
tilia, and those of the Anomodontia; for the capitular and tubercu- 
lar processes are confluent on the former, and widely separated in 
the latter, the tubercular being elevated to the neural arch. Inthe 
Ichthyopterygia they are separated, but both are on the centrum. 
Thus the Reptilian affinities are divided between the Anomo- 
dontia and Rhynchocephalia, and are not very close to either. 
They are much less with the Lacertilia, and still less with the Tes- 
tudinata and Crocodilia. 
There are some extra-reptilian indications worth observing. The 
most important of these is the great extent of the pterygoids back- 
wards and inwards, paralleling only in this some Batrachia, e. ¢., 
Rana (fig. 21, Pt). The large size and form of the stapes are 
similar to that seen in Cecilia. The posterior development of the 
squamosal is alluded to later, in the discussion of the homologies 
of that bone. 
