230 B. NATURAL HISTORY. 
Reptilia, not only as an illustration of their meaning, but of their 
use. 
B. On the System of Reptilia. 
The points of resemblance to the other classes of Vertebrata 
presented by the Reptilia are, of course, to those below them and 
those above them. Relationships to the class Batrachia are as yet 
doubtful, unless indeed the remarkable relations of the squamosal 
and quadrate in Anomodontia have such a significance. The ex- 
tremities of the genus Ichthyosaurus present a remarkable struct- 
ure not seen elsewhere in the class, nor in the classes above it; 
viz., in lacking all differentiation between the elements external to 
the proximal element,—the humerus and femur. So far as form 
is concerned, the ulna and radius, tibia and fibula, tarsus carpus, 
metacarpus, metatarsus, and phalanges, are identical. This type is 
only found below the Reptilia, approximately among Crossoptery- 
gian fishes and Elasmobranchi; and it is to the latter class that we 
must appeal, says Gegenbaur, for an explanation of their structure. 
No other resemblance of real importance has been observed to 
exist between the two groups. 
The extension downwards of the eauanioel over the quadrate 
region constitutes a point of remote resemblance to the Fishes. 
The ? continuation of the ? frontals to. the premaxillaries in Ich- 
thyosaurus is seen in the lower tailed Batrachia. 
Resemblances to the classes above the Reptilia are seen in the 
groups Crocodilia, Dicynodontia, Ornithosauria, and Dinosauria. 
In the first, the presence of a vermis in the cerebellum, and quad- 
ripartite heart are points of equal affinity to the Mammalia and 
Aves. In the three others, the double-headed ribs, with capitular 
articulation on the centra of the vertebre, and generally elongate 
or complex sacrum, are points of resemblance both to Mammalia 
and Birds. In the Dicynodontia, other resemblances to either 
class are wanting, but the case is different in the other orders. The 
pelvis and hind limbs of the Dinosauria are especially bird-like ; 
while, according to Seeley, the Ornithochire had epipubic or 
marsupial bones as in Mammalia, a brain with infero-lateral optic 
lobes as in Aves, and even confluent metatarsi as in the same class. 
In fact, it seems quite evident that Seeley is right in referring that 
group to the Birds; but this does not necessarily remove the true 
Pterodactyles from the Reptilia. These have distinct tarsals and 
metatarsals, though their epipubic (marsupial) bones and other 
characters ally them most closely to the Ornithochire. 
