RODENTIA SACCOMYINAE PEROGNATHUS. 



417 



appearing between the condyloid process and the lower angle, makes its appearance quite 

 prominently outside of the bottom of the notch separating the condyloid and coronoid 

 processes. 



The molars are all rooted throughout life. Each one, as far as can be judged, has four 

 roots ; at least this is the case with the second and third. The molars are slightly convergent 

 anteriorly, their inner lines being straight. In the unworn teeth the crowns are uniformly 

 studded with distinct regular tubercles. In the upper jaw the first molar has four such 

 tubercles, one anterior, one posterior, and one on each side. The second and third molars have 

 two series of tubercles, exactly transverse to the crown, each series consisting of three tubercles 

 of nearly equal size, separated by a transverse valley. A similar arrangement may be seen in 

 the last molar, but in consequence of its diminished size the tubercles become indistinct. The 

 same conditions prevail in the lower jaw, except that the four tubercles of the anterior molar 

 are arranged two anterior and two posterior, and the outer tubercles of the rest are rather low ; 

 indeed, not more than four can distinctly be discerned in the posterior molar. The last molars 

 of both jaws are less than the second and third, which are nearly equal. The first molar is 

 larger than the fourth ; in the upper jaw it is nearly equal to the second ; in the lower it is 

 decidedly less. 



The worn teeth, however, present entirely a different appearance, resembling more closely 

 those of Dipodomys. The tubercles are ground off, leaving two parallel narrow islands of 

 dentine enclosed by enamel, the centre of the tooth being thus traversed transversely by 

 enamel, which is obscurely divided into two lines by a faint channel, the bottom of the trans 

 verse valley between the tubercles already mentioned. Still further wearing would remove this 

 partition, throwing the whole dentine into a single island ; of this condition, however, I have 

 not seen specimens. The deciduous premolars do not differ materially. 



In the following table I have endeavored to show the points in which Perognathus and Dipo- 

 domys differ from each other, as well as from most other genera of Rodents, 



There appear to be two quite distinct groups in this genus, one having the original 

 P.fasciatus for its type, the other probably including the Cricetodipus parvus of Peale. They 

 may be characterized as follows : 



Perognathus. Ears rather large ; a distinct sub-orbicular lobe to the antitragus. Soles 

 naked from the heel. Tail quite densely hairy, sometimes tufted at the tip. 

 53 L 



