064 PROCEEDINGS OF THE NATIONAL MUSEUM. 



in its anterior face. Each genus has three upper molars on a side and 

 as many ])eloAv, with the enamel thrown into folds, which are more 

 complex in Z</j>a.s. Z(/j»/.^ has a small upper premolar, but in the 

 g-eims Nitjxuoziijnis this tooth is entirely wanting. The pi-emolar is 

 lacking in Dq)>ix. 



The lower jaw of 'Anprn is much deeper The lower jaw of l}\pm i.s .shallow be- 



behin.l and has a well developed coro- hind and with the coronoid process 



noid process alnio^^t equaling in size the scarcely at all developed, with a corre- 



condyloid process. The sigmoid notch is sponding diminution of tlie sigmoid 



correspondingly deep ami pronounced. notch. 



Scarcely any prominence can l)e seen The cap covering the root of the lower 



in Zapm corresponding to the covering inci.<or forms a i)rominent i^rujection be- 



of the root of the lower incisor. neath the condyloid process in JVipa^. 



The angle of the lower jaw in Zrt/>H.v is The angle of "the lower jaw in mjnis is 



deepene<l, with the lower liorder turned shallow and perforated by a large fora- 



inward, and is not ])erforated by a fora- men. 



men. 



Alactaga very closely resembles iJlpus and differs from Z((2:nis in 

 essentially the same points that Dijms does. Its chief differences from 

 Bi2)m are the scarcely inflated mastoid l)ulla> and the incomplete fora- 

 men for the nerve at the lower inner angle of the antorbital foramen. 

 The vertical part of the malar is not so greatly expanded laterally and 

 the audital bulhe are less inflated. Alacfuga has the "cannon ''bone 

 oiBipus, ])ut on either side of it is a small non-functional toe, consist- 

 ing of a metatarsal and a digit. The cervical vertebrae show a tendency 

 toward consolidation, but not that complete fusion found in Dipus. 



In AJacfiK/d the incisors are ungrooved and are not recurved as in 

 Dijym, but project more forwai-d. presenting an appearance seen in 

 the Hares. A small premolar is present. The molars have a more 

 complex enamel pattern. 



In nearly all these respects DipuH is seen to be a much more spe- 

 cialized type. Both Dipux and Ahicfxc/a share nearly everything in 

 common, aside from greater specialization, and are placed in strong 

 contrast to Zapm. 



Fuehoreutts^ is an animal with the foot structure of Alactar/a and 

 a skull on the B'q^n.^-AJaetaga type, l»ut appearing more slender and 

 with greatly enlarged bullje. In the structure of its zygomatic arch, 

 as well as in its narrower proportions, it approaches slightly the type 

 of skull seen in Z(ii»t>< and Srninflim. ''The zygoma is very weak 

 and thin, and the vertical portion, which separates the optic from the 

 antorbital formen, is also very thin and slopes from above downward 

 posteriorly (as in Zapux, Sm'n^thm, and the MurUhi^, while mAJadaga 

 the corresponding part of the zygoma is either vertical or anteriorly 



' From the descrii>tion and figures, Sclater, Proc. Zool. Soc, London, 1890, pp 

 610-613. 



