SACCOMYID.E— PEKOGNATHIDIN^E— PEROGNATIIUS. 499 



on the interior side; the back upper molar is simply single-rooted. The 

 under molars have each a pair of roots, aligned lengthwise in a single series, 

 1 >ut the two roots of the back lower molar are imperfectly distinguished. 

 With these last exceptions, each root of all the teetli has its own distinct 

 socket in the alveolus. 



In the perfectly unworn state, the crowns of the molars are studded with 

 tubercles in regular transverse series. In the upper jaw, the anterior molar 

 has four, — an anterior, a posterior, an exterior, and an interior, with perhaps 

 another one part way up the anterior lobe. The second and third upper 

 molars have each six tubercles, in two straight transverse rows of three each, 

 these rows separated by a deep sulcus. The smaller cir- 

 cular back upper molar tends indistinctly to a similar state. 

 In the lower jaw, the tuberculation is very similar; but 

 the four tubercles of the first molar are in an anterior and 

 posterior pair, and on the last one the tubercles become 

 indistinct. The teeth present a very difTe rent aspect when 

 the tubercles are ground off with wear. Each transverse 

 row of tubercles becomes converted into an island of 

 dentine, there being thus, on the intermediate molars at 

 any rate, a pair of such transverse dentine islands separated 

 by a double ridge of enamel partition, between which is 

 the bottom of the sulcus already mentioned. This enamel 

 fold makes in from the outer side of the tooth nearly 

 to the inner side. The front molar shows a little isolated 

 island of dentine anteriorly, nearly circular, and a broad 

 transverse one posteriorly. The state of the under teeth 

 is substantially the same. „ , . . 



J Explanation of fifjs. — 



The upper incisors are small, compressed, with a Left ear - in eaob case > 



. twice the natural size: 



strong backward set. Their face is deeply channelled upper fig. p. mmttcola; 

 \v itfa a longitudinal groove, and the exterior moiety is rab- ? idd ! e fig ' f- ^""f* 



° J lus ; lower fig. Cncetoat- 



beted down so that the groove is visible laterally as well pusflavus. 

 as from the front. This is a prominent character (shared by Cricetodipus and 

 Dipodomys) in distinction from Heteromys. The under incisors are small 

 and simple. 



Before leaving this portion of the subject, I may as well mention a curi- 

 ous circumstance: the ease with which the skulls of Perosnathus and dice- 



