THE GEOLOGICAL MIDDLE AGE. 159 
ball was very large, as may be seen by the socket, 
and it was supported by pieces of bone, such as 
we find now only in the eyes of birds of prey and 
in the bony fishes. The ribs begin at the neck 
and continue to the tail, and there is no distinc* 
tion between head and neck, as in most Reptiles, 
but a continuous outline, as in Fishes. They had 
four limbs, not divided into fingers, but forming 
mere paddles. Yet fingers seem to be hinted at 
in these paddles, though not developed, for the 
bones are in parallel rows, as if to mark what 
might be such a division. The backbones are 
short, but very high, and the surfaces of articula- 
tion are hollow, conical cavities, as in Fishes, in- 
stead of ball-and-socket joints, as in Reptiles. 
The ribs are more complicated than in Verte- 
brates generally: they consist of several pieces, 
and the breast-bone is formed of a number of 
bones, making together quite an intricate bony 
network. There is only one living animal, the 
Crocodile, characterized by this peculiar struct- 
ure of the breast-bone. The Ichthyosaurus is, 
indeed, one of the most remarkable of the syn- 
thetic types : by the shape of its head one would 
associate it with the Porpoises, while by its pad- 
dles and its long tail it reminds one of the whole 
group of Cetaceans to which the Porpoises be- 
long ; by its crocodilian teeth, its ribs, and its 
breast-bone, it seems allied to Reptiles ; and by 
