VISCERA. HABITS. 631 



cheeks. The stomach is generally simple, but there may be 

 complications. It may be constricted between the cardiac 

 and pyloric portions with a groove leading from the oesophagus 

 to the pyloric end and occasionally (e.g. Castor) there are 

 cardiac glands. In Myoxus the oesophagus has a glandular 

 dilatation at its lower end. There is always a large caecum 

 except in Myoxus and the gall bladder is generally present. 

 The colon is occasionally spirally twisted. There are two 

 superior venae cavae. In some genera the ureters open into 

 the fundus of the bladder. Vesiculae seminales are present 

 except in the Dupliciclentata and the penis usually has a bone. 

 The testes are in most cases abdominal, descending into the 

 inguinal region in the breeding season. In the Duplicidentata 

 they may be said to descend into a scrotum in which they 

 remain permanently. A prostate is present. The uterus is 

 bicornuate, or may be quite double. They usually produce 

 numerous young at a birth and have a considerable number 

 of thoracic and abdominal mammae. The young are born 

 naked and helpless in the burrowing forms. The placenta 

 and the chorion are both discoidal. 



Odoriferous glands are very generally present ; they open 

 into the prepuce or into the rectum or near the anus. 



The Rodentia are the largest mammalian order. There 

 are about 1,400 species and 160 genera. They are cos- 

 mopolitan. They are most numerous in South America, 

 which has been termed their home, and least so in Australia 

 and Madagascar, where they are represented by only a few 

 genera of Muridae. 



They present great diversity of life. They all gnaw and 

 are herbivorous. A few are omnivorous. It is possible that 

 one or two may be carnivorous. Some are arboreal, and in 

 the flying squirrels provided with a parachute-like membrane 

 for floating in the air. Others are aquatic, as the water-vole, 

 while yet others live on the earth and are strong runners, as 

 the hares. Many of them build nests, dig out complicated 

 burrows, and lay up stores for the winter. The latter usualh' 

 possess cheek pouches. Some fall into a winter sleep at the 

 cold season of the year. 



Typical rodents are first found fossil in the Upper or 

 possibly in the Lower Eocene, most of the remains belonging 



