482 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1907. 
carnivorous, the group of herbivorous sea-cows is sharply separated. 
Their ancestors were not carnivorous but herbivorous mammals, and 
they have remained true to this regimen. 
Il. Tur Mesozoic Marine REPTILES. 
Since the ancestors of the whales, of the sea-cows, and of the seals 
can not be looked for among the living mammals, the question may be 
raised whether the Mesozoic marine reptiles may not be regarded as 
their ancestors. Indeed, on superficial observation, the well-known 
ichthyosaurs (fig. 9) presents the form with which we are familiar in 
the dolphin. The body is fish-like, the skull, as in the dolphin, extends 
into a long snout with numerous teeth, the limbs have the form of 
flippers, there is a dorsal fin which reminds one forcibly of that of 
a dolphin, the skin is naked, and the young are born alive. 
PARAS AO Ne 
Fic. 9.—Restoration of Ichthyosaurus quadriscissus Quenstedt from the Upper Lias of 
Wiirttemberg. Length about 2 meters (63 feet). After O, Jackel, 
Opposed to these similarities of form, however, are many much 
more important structural differences. The ichthyosaurs were rep- 
tiles which were not related to the mammals in the remotest degree, 
and became extinct without leaving descendants. 
On more careful observation, we see, moreover, that the caudal fin 
in the ichthyosaur is not horizontal, as in the whales, but vertical, 
as in the fishes. In addition, not only the fore limbs but the hind 
limbs are transformed into fins. Only the form of these fins can be 
compared with those of whales; their structure is entirely different. 
The similarities between ichthyosaurs and dolphins can not, there- 
fore, be looked upon as evidence of relationship. They result from 
a similarity of adaptation for the same mode of life. If we search 
among the other marine reptiles which took the place of whales in 
Mesozoic times, we do not meet with any form which exhibits any 
similarity to living mammals. In contrast with the dolphin-like 
ichthyosaur, stands the rotund plesiosaur, with its turtle-like body, 
four long fins, and a small skull resting on a very long neck. With 
