MAMMALS. 405 



recent forms. In place of teeth, horny plates are developed in 

 the palatal region and at the front of the lower jaw, and these 

 are masticatory in function. The stomach is divided into two 

 principal chambers, and these in turn may be sacculated. The 

 diaphragm is oblique, the lungs very long, and the heart is bifid 

 at the tip, the two ventricles being partially separated (Fig. 352). 

 The testes are abdominal in position ; the uterus is two-horned. 

 The placentation of the dugong alone is known. This form has 

 a non-deciduate zonary placenta. 



The living species of sea-cow are few. All are littoral in 

 their habits, but never leave the water. They feed upon sea- 

 weed, or upon the grasses growing in the rivers. They are per- 

 fectly harmless, although they attain considerable size. These 

 animals may afford the grain of truth in the mermaid myth. 



The PRORASTOMID.E (only genus Promstoiiis from the eocene of 

 Jamaica), is Icnown only from the skull. It is remarkable in having a com- 

 plete dentition : z |, c \, p ^, >n \. The MANAxro^ have molars 8 to 10, the 

 first single-rooted; incisors and canines never functional. Manatus {Tri- 

 chechus') includes the manatees of tropical America and Africa. Fossil in 

 pleistocene of South Carolina. They have but six cervical vertebras. Here 

 possibly belong the tertiary Manaihermm and Ribodon. HALICORID.E with 

 heterodont molars, and either with tusk-like incisors in the upper jaw or these 

 lacking. Halicore includes the dugongs of the Indian Ocean. Halit/iei'iurn, 

 from the miocene and pliocene of western Europe, gives evidences of a milk 

 dentition. Rhytoidus, Felsinotherm7n, from the tertiary. RhytindIjE, with- 

 out teeth. The only known species, Rliytina stelhri, the northern sea-cow 

 of the northern Pacific, was exterminated in the last century. 



ORDER VII. CETACEA. 



Aquatic mammals without distinct neck ; fore limbs paddle- 

 like ; hind limbs absent ; usually a dorsal fin ; caudal fin in two 

 lobes or ' flukes,' nostrils on the top of the head ; teeth, when 

 present, homodont and monophyodont ; no elbow joint ; brain 

 large, cerebrum complicately convoluted ; placenta non-deciduate, 

 diffuse. 



The whales, like the sea-cows, form a distinctly circum- 

 scribed group, sharply marked off from all others, so that no 

 clear conclusions can be reached as to their line of descent from 

 other groups, although one is justified in believing that they 



