AQUATIC MAMMALS 



which unquestionably was utilized for anguilliform propulsion, and the 

 other by Zeuglodojz osiris, with a rather short tail suitable for fusiform 

 propulsion. Just as these two swimming types grade one into another 

 in the case of fish, according to body length and slenderness, so may 

 we presume that there were zeuglodonts at some period that were inter- 

 mediate between the two types mentioned. Basilosaurus, however, certain- 

 ly could not have had flukes like modern cetaceans, for it had not the 

 musculature to control such equipment, as indicated by the low spinous 

 processes. But the tail was very mobile, because the zygopophyses did 

 not articulate, and it must have had some sort of caudal expansion, but 

 of great lineal rather than lateral extent. The same applies in some 

 measure to Zeuglodon osiris, except that having a short tail its caudal 

 expansions must have been lineally shorter and possibly somewhat sug- 

 gestive of the manati. 



Modern whales are exclusively aquatic. The body is fusiform, with 

 no real constriction at the greatly shortened neck, and tapers gradually 

 from the thorax to the base of the tail. The latter terminates in a pair 

 of flukes, vertically flattened and laterally expanded into two pointed 

 lobes with a notch between them. There is no vestige of external hind 

 limbs and the anterior limbs are paddle-shaped, the integument being 

 continuous over the digits, which have no sign of nails, and there is no 

 external division of the limb into segments. The skin is smooth and 

 hairless except that there are occasionally a very few bristles about the 

 head. There are no sebaceous glands and the suderiferous ones are ab- 

 sent or much reduced. There is a specialized blubber layer beneath the 

 skin. The eyes, which are specialized to function in salt water, have 

 no nictitating membrane nor lachrymal duct. There is no pinna to the 

 ear and the auditory meatus is a minute aperture. The nostrils open 

 not near the snout (save in sperm whales) but near the vertex of the 

 head, with corresponding alteration in the bones of the skull. The lat- 

 ter is very remarkable in that some of the bones have slid partly over 

 others, resulting in the condition referred to as telescoping. The bones 

 are spongy, the cavities being filled with oil. The cervical vertebrae 

 are often phenomenally shortened and in some species most or all of 

 them are fused. The articular connections of the body vertebrae are 

 much reduced and are separated by elastic intervertebral discs affording 

 great freedom of movement. The spinous processes are always well de- 

 veloped. There is no sacrum and the body and caudal vertebrae are dif- 

 ferentiated only by the presence upon the latter of chevrons. The ru- 

 dimentary pelvis has no contact with the vertebral column. There are 



[42] 



