334 LIMBS OF SIRENIA 



like or rhomboidal to circular in form. Nostrils on upper surface 

 of not specially -elongated snout. Clavicles are absent. The 

 scapula has the normal mammalian form, with a well-developed 

 and roughly median spine. The bones of the arm and hand articu- 

 late together, as in land animals ; the phalanges show at most 

 traces of increase in number above the normal. Pelvis represented 

 by a vestige, more highly developed in some fossil than in recent 

 forms. Stomach complex, consisting of several chambers. Lungs 

 simple and not lobulated. Diaphragm ol:)licpie and very muscular. 

 Brain peculiar in form and l.)ut slightly convoluted. Testes ab- 

 dominal. Teats two, and pectoral in position. Placenta non- 

 deciduous and zonary.i 



This limited group consists of purely aquatic forms, which are 

 both marine and fresh-water in their proclivities. They have 

 been placed in the immediate vicinity of the Whales ; but it is 

 now believed l;)y most zoologists that the likenesses which they 

 undouljtedly show to the Cetacea are of an adaptive kind and 

 related to tlieir similar mode of life. The group is a readily- 

 definable one. Externally they are marked by tlieir dark 

 coloration, somewhat Whale-like though of clumsier build, and by 

 the total absence of external ears and hind-limbs; the latter are, 

 however, as will 1)6 pointed out shortly, marked by certain rudi- 

 mentary bones. There is a flattened tail, which in the Dugong and 

 Rhytina is precisely like that of a Whale. It is interesting to 

 note that the former genus, whose tail is, judging it at least by 

 the standard of the Whales, more completely modified for the 

 aquatic life, should also show other features which indicate their 

 longer life as marine creatures. For the flippers are more Whale- 

 like in that the fore-arm is completely enclosed within the body, 

 or nearly so, and the nostrils have a more decidedly superior 

 position than in the Manatee. The fore-limbs of this group, as 

 may be inferred from what has just Ijeen said, are flipper-like ; 

 but, contrary to what we find in Whales, the phalanges do not as 

 a rule show any traces of multiplication, so characteristic a feature 

 of the Cetacean hand, and the individual bones are connected by 

 well-formed joints. Beneath the thick skin, which is sparsely 

 provided with stout hairs in the Dugong, is a layer of blubber. 

 Dr. Murie has called attention to the fact that this layer in the 



^ In Halicorc ; j)robably also in Manatus. Se^iTuvney, IVans. Jloy. Soc. Edinb. 

 XXXV. 1889, p. 641. 



