370 THE ANATOMY OF VEETEBRATED ANIMAIiS. 



either smooth externally, or very moderately convoluted. 

 The coq3us callosum is well developed. 



With the exceptions noted, the foregoing characters are 

 universal among the Uodentia. There are other peculiarities 

 which are generally present, and, when they exist, are very 

 characteristic, though they are not universal. 



Thus the dorso-lurabar vertebrae are usually nineteen in 

 number. There is a large interparietal ossification. The 

 jugal bone is comparatively short, and occupies only the mid- 

 dle of the zygomatic arch. 



•The clavicles are very generally present; though wholly 

 absent in some genera, as, for example, the Guinea-pig, 

 ( Cavia). The acromion commonly sends a process backward 

 over the infra-spinous fossa. There is a ninth bone in the car- 

 pus intercalated between the proximal and the distal series. 

 The digits are five, ungulate, and provided with small claws. 



There is a bone in the penis. The testes do not leave the 

 abdomen, but come down into the groin in the breeding-sea- 

 son. Vesiculce serainales and prostatic glands are present. 

 In the female the uterus is, in many genera, completely di- 

 vided into two cornua, each of which opens separately into 

 the vagina ; but, in the rest, the cornua unite into a corpus 

 uteri. 



Some genera depart widely from the rest in particular 

 points ; for example, in the Porcupines, the hairs on the dor- 

 sal region of the body are very much enlarged, acquire a pe- 

 culiar structure, and formed the so-called " quills." Some of 

 the Porcupines have prehensile tails. 



In Cavia and Uijdrochoerus the toes are reduced to three, 

 and the nails have almost put on the character of hoofs. 



The Squirrels have the short pollex almost opposable. 



The femur in some Rodents has a well-developed third 

 trochanter ; and in D'lpus^ the Jerboa, the long metatarsals 

 become anchylosed together into a cannon-bone. 



In the Porcupines, the suborbital foramen is enormous, 

 and an anterior fasciculus of the masseter muscle arises fi'om 

 the maxilla, and traverses the foramen to its insertion. 



The Hamster ( Cricetus) has great cheek-pouches, provided 

 with special retractor muscles connected with the spines of 

 two lumbar vertebnr. 



In some genera, the stomach, which is usually simple, tends 

 to become complex. Thus the cardiac division of the stomach 

 of the Beaver is provided with a special glandular mass. The 

 cardiac end of the oesophagus of the Dormouse is glandular 



