ALIMENTARY TRACTS AND ADJACENT GLANDS 



91 



the mouth may be divided into two areas and classes, as 

 follows : 



The diastemal ridging occurs in the space between 

 the incisors and the molariform teeth. In Homodontomys 

 there is a narrow ridge parallel with the long axis of the 

 skull extending from just caudad of the incisors. Poste- 

 riorly it enlarges to terminate in an eminence marked by a 



Fig. 19. A, Roof of the mouth of Teonoma with lower jaw removed, 

 showing ridging, (a) sphincter pads, and (6) oral bristle areas. 



B, Stomach of Teonoma, showing (a) oesophagus, (b) duodenum, (c) 

 pyloric valve, (d) cardiac horn, (c) pj'loric horn, (/) position of fundus 

 gland. 



slight, transverse crest. In sequence caudad to this there 

 is a narrow, transverse ridge and two broader ones, the 

 former of these two being obtusely angular. In Neotoma 

 the caudal termination of the first ridge is larger, and in 

 Teonoma, larger still, being in reality three parallel ridges 

 joined at their bases. This variation in ridging is in re- 

 sponse to slight differences in the shape of the oral sphinc- 

 ter pads of the respective subgenera, for the pads, through 

 medial pressure, have molded the anterior ridging of the 

 roof of the mouth. 



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