THE RODENTIA. 3G9 



spreads over the interior of the chorion, and gives rise to the 

 broad zone-like placenta, which is composed of both maternal 

 and foetal parts. The maternal vessels pass straight through 

 the thickness of the placenta toward its foetal surface, on 

 which they anastomose, forming meshes, through which the 

 vessels of the foetus pass toward the uterine surface of the 

 placenta. 



The species of the genus Syrax are found only in Syria 

 and Africa. No fossil Hyracoidea are known. 



The DiscoiDEA. — The Mammalia with discoidal placentae 

 are the .Rodentia^ the Cheiroptera^ the lyisectivora^ and the 

 Primates. 



1. The RoDENTiA. — This large groujD of Mammalia is most 

 definitely characterized by its dentition. There are no canines, 

 and the mandible never contains more than two incisors, which 

 are placed one on each side of the symphysis, and continue to 

 grow throughout life. They are coated with enamel much 

 more thickly upon their front surfaces than elsewhere ; so 

 that by attrition they acquire and retain a chisel-shaped edge, 

 the enamel in front wearing away less rapidly than the rest 

 of the tooth. 



With the exception of one group of Rodents, there are only 

 two teeth in the premaxillse ; and these have the same char- 

 acters as the incisors of the mandible. The Lagomorplia^ or 

 Hares and Rabbits, however, have a second pair of incisors 

 of small size, behind the first, in the upper jaw. The molars 

 are from two to six in number, in each half of the upper jaw, 

 and two to five, in the lower jaw. They consist of enamel, 

 dentine, and cement, and their crowns may be tuberculate or 

 laminate in pattern. Sometimes they form roots, but, in other 

 cases, they grow throughout life. Where there are more than 

 three grin ding-teeth, the one which precedes the three hinder- 

 most has displaced a milk-tooth ; but, where the grinding- 

 teeth are fewer than three, or only three, none of them dis- 

 place a milk-tooth. Even when milk-teeth exist they may 

 be shed before birth, as in the Guinea-pig. 



The premaxillary bones are always large, and the orbits 

 are never shut off by bone from the temporal fossa. Very 

 generally, the condyle of the mandible is elongated from be- 

 fore backward. 



With the exception of one group, the Dormice [3fyoxince)^ 

 all Rodents have a lar2:e caecum. 



The cerebral hemispheres leave the cerebellum largely 

 uncovered, when the brain is viewed from above. They are 



