INTERRELATIONSHIPS OF GENERA 



71 



In the skull there are two primitive features; the palate is 

 abnormal posteriorly, lacking postero-lateral bridges, having the 

 postero-median septum represented merely by a slender and 

 free spinous process, and having the postero-lateral pits small 

 and ill defined ; the auditory buUaj are small and lack internal 

 spongy tissue, although the external meatus is distinctly 

 tubular, and the stapedial artery is enclosed in a bony 

 tube as far as the stapes. The mandible is nearly normal, but 



Fig. 44. — Ondatra zibethica Linnseus. 

 Dorsal view of skull (enlarged). 



it shows one primitive character ; the lower incisor passes back- 

 wards below the molar roots to terminate in the lower part of 

 the condylar process, displacing m^ lingually to a very slight 

 extent. The upper incisors are normal, without grooves, and 

 terminate in the maxilla immediately in front of the anterior 

 root of m^ as usual (Fig. 21). 



In all other respects Ondatra is greatly modified for its peculiar 

 habits. Its size has greatly increased beyond that of its terrestrial 

 relatives. The fur is waterproof, made up of very dense, long, 

 fine, and silky under fur, overlaid and more or less completely 



