MUR1DAE— SIGMODONTES— HESPEROMYS. 49 



In Hesperomys, as in Mus and Ochetodon, and not as in Sigmodon and 

 Neotoma, the tubercles of the molar crowns are long persistent. A great 

 majority of the specimens of Hesperomys in the collection before us present the 

 tubercles intact, while it is rare to see skulls of Neotoma and Sigmodon in 

 which the crowns are not already ground flat, so as to show the dentine area 

 surrounded by the plicated enamel-sheet. This would seem to argue a much 

 slower growth of the grinders. In the rapidly and continuously growing 

 molars of Arvicola, the crowns are worn flat, and show their characteristic 

 dentine triangles as soon as they fairly surmount the alveoli ; here the oppo- 

 site extreme is witnessed. The unworn molars of Hesperomys show a double 

 lengthwise series of conical tubercles connected by lower crosswise ridges, 

 and the whole face of the tooth is encased in a sheet of enamel continuous 

 with that of the sides of the tooth. Although, as we have said, the main 

 tubercles are biseriatim, yet the first pair of the front molar of either jaw may 

 appear like one, from being so close together ; this azygos anterior one being 

 followed by two perfectly distinct pairs; the second tooth has only two pairs, 

 but perfectly distinct ones ; on the small hinder tooth, the pairing of the tuber- 

 cles is obscure. The tubercles are not exactly opposite each other in crosswise 

 pairs, but are half-alternating. Down between the bases of these conical 

 eminences are seen furrows, the more readily noticeable because generally 

 blackened, apparently by the sticking of foreign matter in them. They rep- 

 resent the deep close-curved plications of enamel that penetrate the tooth 

 from either side ; the ends of the loops nearly or quite meeting in the substance 

 of the tooth. 



If the foregoing account is perfectly intelligible, it will be seen that, after 

 abrasion has commenced, the molar crowns will present a different pattern 

 with each stage of the process. The main conical tubercles are first razeed, 

 and then the connecting crests and little accessory tubercles follow by the 

 same filing-down operation ; consequently, the pattern of the molar crowns 

 must be used as a zoological character with great caution, if at all ; the minor 

 details are of no sort of consequence; and even in using this broad pattern it 

 is necessary to compare age for age (or rather condition for condition, since 

 different individuals get their teeth filed down with variable rapidity) in draw- 

 ing up the characters of species or subgenera. At the last stage specified, 

 namely, when main tubercles and connecting crests and accessory tubercles 

 have disappeared, we see a single dentine area occupying the whole face of 

 4 M 



