434 THE ANATOMY OF VEETEBEATED ANIMALS. 



part of the Eustachian tube is dilated into a tliin-walled 

 sac extending on the inner side of the bulla tympani from 

 the pterygoid processes to the exit of the ninth nerve. 



A slight constriction marks off the cardiac from the 

 pyloric division of the stomach. The cardiac portion is 

 lined by a dense epithelium. The intestine is provided with 

 thi'ee coeca — one in the ordinary position, and two placed 

 much lower down on the colon, opposite one another, and 

 tenninating by pointed ends. There is no gall-bladder. 

 The ureters oj)en, not near the neck of the bladder as in 

 Mammals generally, but near the fundus, as in some few 

 Rodents. 



The male has vesiculce seminales, prostatic and Cowperian 

 alands. The uteiiis is two-homed, and the vulva and anus 

 are surrounded by a common fold of integument. 



In the foetus the yelk sac and the vitello-intestinal duct 

 early disappear. The amnion is not vascidar. The allantois 

 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 towards its 

 foetal siu-face, on which they anastomose, forming meshes, 

 through which the vessels of the fcetus pass towards the 

 uterine surface of the placenta. 



The species of the genus Hijrax are found only in Syria 

 and Africa. No fossil Hyracoidea are known. 



The Discoidea. — The Mammalia with discoidal placentae 

 are the Bodentia, the Cheiroptera, the Lisectivora, and the 

 Primates. 



1. The RODENTIA. — This large group 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 acquii'e 

 and retain a chisel-shaped edge, the enamel in front weaidng 

 away less rapidly than the rest of the tooth. 



