PLACENTAL MAMMALIA. 



763 



(2192.) The lowest order of PLACENTAL 

 MAMMALIA comprises those forms which, 

 although they breathe air by means of lungs, 

 and have hot blood like ourselves, are ap- 

 pointed to inhabit the waters of the ocean, 

 wherein they pass their lives, and even 

 bring forth and suckle their young. In 

 order to live under such circumstances as 

 these, the CETACEA must necessarily, in 

 many points of their structure, be organized 

 after the model of fishes ; and we cannot be 

 surprised if, in their outward form, and 

 even in the disposition of their limbs, they 

 strikingly resemble the finny tribes. Their 

 head is large frequently, indeed, of enor- 

 mous proportions; there is no neck appa- 

 rent externally, the head and trunk, as in 

 fishes, appearing continuous. The anterior 

 extremities are converted into broad fins, or 

 paddles ; whilst the pelvic extremities are 

 absolutely wanting: posteriorly, the body 

 tapers off towards the tail, and terminates 

 in a broad, horizontal fin, which latter, 

 however, is not supported by bony rays, as 

 in the fish, but is entirely of a cartilaginous 

 and fleshy structure. Frequently there is 

 even a vertical dorsal fin ; but this, too, is 

 entirely soft and cartilaginous, so that in 

 the skeleton no vestiges of it are apparent*. 



(2193.) In the Whalebone- Whale (Ba- 

 Icena mysticetus) the peculiarities of the 

 Cetaceous skeleton are well exhibited. In 

 this gigantic animal (fig. 385), which some- 

 times measures upwards of a hundred feet 

 from the snout to the tail, the head forms 

 nearly a fourth part of the entire length of 

 its stupendous carcass ; so enormously de- 

 veloped are the bones of the face that form 

 the upper and the lower jaws. The cranial 

 cavity, wherein the brain is lodged, of 

 course does not participate in this excessive 



* It is interesting to see these fins still formed by 

 the skin (exosJcdeton], where the osseous system 

 could not enter into their composition without de- 

 viating altogether from the Mammiferous type. 



Fig. 385. 



Skeleton of the Whalebone 

 Whale. 



