Dr. C. W. Andreivs — Prozeuglodon atrox. 211 



from terrestrial mammals during the Lower Eocene period, and long 

 before the close of the Middle Eocene had become almost as completely 

 adapted to an aquatic life as their descendants at the present day. 

 It may be of interest to consider the reasons for this rapid change. 

 In the first place it is pretty certain that at the close of the Mesozoic 

 period all the groups of great marine reptiles had, for some unknown 

 reason, become extinct, so that, excepting the fishes and a Hhyncho- 

 cephalian, no vertebrates inhabited the early Eocene seas. The con- 

 sequence of this would be that if any terrestrial forms should adopt an 

 aquatic life, the freedom from competition and, to some extent, from 

 powerful enemies, would offer exceedingly favourable conditions fortheir 

 rapid spread and multiplication in the seas, while the great development 

 and differentiation of the land mammals at the same period would render 

 the conditions of terrestrial life more and more exacting. Consequently, 

 when the primitive swamp-loving Proboscidean ancestor of the Sirenia 

 and the early Creodont from which the Zeuglodonts arose took to 

 living in the water, everything was in their favour. The rapidity 

 with which these animals became adapted to their new conditions of 

 life is probably the direct result of the complete change in the 

 mechanical conditions of life. Eor one thing, the limbs cease to 

 support the weight of the body, which is now borne by the water 

 in which it floats. On the other hand, the limbs are still used for 

 propulsion, and consequently there is a forward thrust at their 

 proximal ends. The consequence of this thrust in changing the 

 structure of the pectoral and pelvic girdles has already been discussed 

 in the case of the Plesiosaurs. The manner in which movement 

 through the water is affected of course differs widely in different 

 groups of aquatic animals. Thus, in tbe whales it is by the fore-limb 

 and a transverse tail- fin, the hind-limb being lost ; in the Ichthyosaurs 

 by both the fore and hind paddles, the former being the more important, 

 and a vertical tail-fin; in the sea-crocodiles {Metriorhynchus^ etc.) by 

 the hind-limb and tail ; and in the Plesiosaurs by the oar-like fore and 

 hind paddles. Whatever means, however, is employed to propel the 

 body through the water, during motion it is subjected to a pressure on 

 its anterior end, more or less in the direction of tbe long axis, and 

 during very rapid movement this pressure must be considerable. Most 

 of the peculiarities of the skull of the whales seem to result from this 

 new condition. In the first place, to facilitate passage through the 

 water a more or less slender rostrum is as a rule developed, though 

 in the later and. larger forms of some of the groups it may be lost 

 or masked by other structures. In the next place, the pressure seems 

 to have brought about the spreading back of the posterior ends of 

 some of the facial bones, so that the premaxillse and especially maxillae 

 overlapped the bones behind to a great degree. The same force also 

 may have had much to do with the shifting back of the external nares. 

 The peculiarities in the structure of the cranial portion of the skull 

 •are also capable of explanation in the same way. The primitive 

 whales seem to have had brains of considerable size, and the brain-case 

 would be liable to distortion from continual frontal pressure. This 

 pressure seems, for instance in Phoccena, to have resulted in the 

 ■squeezing out of the parietals from the middle line, and the extension 



