862 PHYLUM CHORDATA : CLASS MAMMALIA 



dorsal position and valvular aperture of the single or double 

 nostril, the sponginess of the bones, the retia mirabilia 

 storing arterial blood in different parts of the body, may be 

 associated with the aquatic life. 



The cervical vertebras are thin, and more or less fused. 

 There is no union of vertebras to form a sacrum, for 

 the hind-limbs are at most very rudimentary. Under 

 the caudal vertebrae there are wedge-shaped chevron 

 bones. 



The brain-case is almost spherical ; the supraoccipital 

 meets the frontals and shuts out the parietals from the roof 

 of the skull ; the frontals arch over the orbit ; the snout or 

 rostrum of the skull is composed of premaxillae, maxillae, 

 and vomer, and of the mesethmoid cartilage. The periotic 

 in whales is an exceedingly dense bone, and is of interest 

 because it is the only part of the skeleton found at great 

 depths on the floor of the ocean, and is often preserved as 

 a fossil. 



There are at least rudiments of two sets of teeth, as 

 in other Mammals, but in baleen whales only the teeth of 

 the milk set are calcified, and they come withal to nothing, 

 being to some extent replaced by the horny baleen-plates 

 developed on the palate. In toothed whales the two sets 

 are said by Kiikenthal to fuse, but the usual interpretation 

 is that the functional teeth belong to the milk set. It 

 is possible that the simple, homodont, conical teeth of 

 Odontoceti have resulted from a splitting of more complex 

 cusped teeth. No clavicles are developed. The bones of 

 the fore-limb are flattened, and, except at the shoulder, 

 articular surfaces are not developed, so that the limbs form 

 stiff paddles. The carpals are fixed in a fibrous matrix, 

 tend to be rudimentary, and are often unossified. They 

 cannot be readily compared with the members of the 

 typical mammalian carpus. In the absence of true joints, 

 a slight flexibility is given by the absence of ossification. 

 There are four or five nailless digits, of which the second 

 and third, and sometimes the first, may have more than 

 the usual number of phalanges (see Fig. 511), a peculiarity 

 possibly due to a duplication and separation of epiphyses. 

 The pelvis may exhibit a rudimentary ischium, with small 

 vestiges of femur and tibia. 



