744 



MAMMALIA. 



The spindle shape, the absence of external ears, the 

 absence of an eye-cleansing nictitating membrane, the 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 asso- 

 ciated with the aquatic life. 



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

 There is no union of vertebrae 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, 



"** / 

 FIG. 366. External appearance of common porpoise. 



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 calcined, 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 



