MAMMALIA. 643 



and vigorous leaps over a level plain ; and in the Kangaroos 

 (Macropus) the extraordinary developement of the posterior extre- 

 mities is even yet more wonderful. In other respects, the skeletons 

 of the Marsupialia conform to the general description already given. 



(723.) All other Mammiferous Vertebrata produce their young 

 alive, and not until they have attained a considerably advanced state 

 of developement during their intra-uterine existence. The con- 

 nection between the maternal and fetal systems in these orders is 

 maintained during the latter periods of gestation by the develope- 

 ment of a peculiar viscus, called the placenta: nevertheless, after 

 birth, the young animals are still dependent upon the mother for 

 support, and live upon the milk supplied by her mammary organs. 



(724.) The lowest order of PLACENTAL MAMMALIA com- 

 prises those forms which, although they breathe air by means of 

 lungs, and have hot blood like ourselves, are appointed 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 enormous propor- 

 tions : there is no neck apparent externally ; the head and trunk, as 

 in fishes, appearing continuous. The anterior extremities are con- 

 verted 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 ver- 

 tical dorsal fin ; but this, too, is entirely soft and cartilaginous, so 

 that in the skeleton no vestiges of it are apparent.* 



In the Whalebone- Whale (Bal&na mysticetus) the peculiari- 

 ties of the Cetaceous skeleton are well exhibited. In this gigantic 

 animal (fig. 299), which sometimes measures upwards of a hun- 

 dred feet from the snout to the tail, the head forms nearly a fourth 

 part of the entire length of its stupendous carcass ; so enormously 

 developed are the bones of the face that form the upper and 

 the lower jaws. The cranial cavity, wherein the brain is lodged, 

 does not of course participate in this excessive dilatation, but 



* It is interesting to see these fins still formed by the skin (exoskeleton,) where the 

 osseous system could not enter into their composition without deviating altogether 

 from the Mammiferous type. 



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