OBSERVATIONS ON THE RODENTIA. 599 



the lower line e e, which is drawn parallel with the grinding 

 surface of the molar teeth, represented by the upper line e e 

 in the figures ; the angle (**) is much elevated, and in fact 

 is situated above the upper horizontal line just mentioned. 

 Now it will be seen upon referring to the figure of the jaw of 

 Ondatra, th at the same characters exist, — the j aw of this animal 

 differs from that found in other rodents in the same manner, 

 here, however, the angle ** is considerably produced, and 

 somewhat twisted outwards ; the transverse diameter is pro- 

 portionately rather less, and the longitudinal greater. In 

 Arvicola and Lemmus, with the same general characters, we 

 find the descending ramus still more twisted ; here the greater 

 portion of this process has assumed an almost horizontal po- 

 sition. The jaw of Spalax differs from that of Arvicola in 

 the comparatively small transverse diameter of the descend- 

 ing ramus, and its greater longitudinal extent; the upper 

 angle is directed outwards, and forms a small nearly semicir- 

 cular platform, which is oblique in its position ; the lower 

 incisor is remarkably long, and extends backwards and out- 

 wards with its thin covering of bone beyond the condyle, so 

 that the jaw appears as if it had a double condyloid process. 

 The apex of the coronoid process is situated above the con- 

 dyle, as in the beaver, in the form and height of the coronoid 

 process however, Geomys approaches still nearer to the bea- 

 ver. The peculiar form of the posterior part of the jaw of 

 Spalax occurs also in Geomys, excepting that here the lower 

 portion of the descending ramus is still narrower, and is 

 merely represented by a slight ridge which runs along the 

 under side of the alveolus of the inferior incisor ; the upper 

 portion assumes the form of a semicircular and nearly hori- 

 zontal platform (the upper surface of which is slightly con- 

 cave), which is situated on the outer side of the alveolus of 

 the incisor, a little below the level of the articular surface of 

 the condyle. As we view the jaw from above, the condyle is 

 the innermost of three processes : the projecting plate, or 

 angle of the jaw just described, being the outermost, and the 

 bony covering of the posterior portion of the incisor being 

 situated between the two. 



The three skulls figured afford types of all the forms which 

 I have met with in the Arvicolidce. The skull of Ondatra 

 resembles that of the water-rat, and that of the lemming 

 [Lemmus norvegicus) differs only in being proportionately 

 broader and shorter, and in one or two points of minor im- 

 portance. Like the species of Arvicola and Ondatra, the 

 lemming has an opening in the temporal bone, situated be- 



