282 PROCEEDINGS OF THE ACADEMY OF 



exceptions, each root of all the teeth 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 ex- 

 terior, 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 tln-ee each, these rows 

 separated by a deep sulcus. The smaller circular 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 different 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 inter- 

 mediate 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 essentially the same. 



The upper incisors are small, compressed, with a strong back- 

 ward set. Their face is deeply channelled with a longitudinal 

 groove, and the exterior moiety is rabbeted down so that the groove 

 is visible laterally as well as from the front. This is a prominent 

 character, shared by Cricetodipus and Dipodomys, in distinction 

 from Saccomys and Heteromys. The under incisors are small and 

 simple. 



Before leaving this portion of the subject, I may as well men- 

 tion a curious circumstance: the ease with which the skulls of 

 Perognathus and Cricetodipus break apart across the middle. 

 This seems to be chiefly due to the delicate state of the zygomata, 

 which afford no stable connection between the fore and aft parts. 

 The break occurs at the basispheno-occipital, squamo-mastoid, and 

 fronto-parietal sutures the parietals, temporals excepting squa- 

 mosals, with the occipital, coming away from the rest of the skull. 



