98 MR. E. R. ALSTON ON THE ORDER GLIRKS. [Jan. 18, 



1. Lepns, Linnseus, Syst. Nat. i. p. 77 (1766). 



External characters those of the family ; all the grinding-teeth 

 with three narrow laminae, except the last lower molar, which is small 

 and simple. 



Fossil genus : — 



2. Palceolagus, Leidy, Proc. Acad. Philad. 1856, p. 89 . (1856). 

 Incisors longer than in Lepus, first lower premolar with only two 



laminae. 



Suhorder III. Glires hebetidentati (suhord. nov.). 



Incisors -f ; the second lower pair very small, and placed rather 

 behind the middle pair, their enamel continuous round the tooth, 

 and their crowns transversely hollowed, not chisel- edged. Grinding- 

 teeth rootless, curved, with their convexity directed outwards. 

 Mandibular condyles and glenoid cavity transverse. Fibula articu- 

 lating with the calcaneum. One family: — 



Family I. MesotheriiDjE. 



Two premolars above and one below ; grinding-teeth rootless, with 

 single reentering enamel-folds. Skull massive, with sagittal and 

 occipital crests enormously developed, the latter running forwards at 

 the sides to the zygoma ; frontals with large postorbital processes ; 

 infraorbital opening small ; malar very deep, running forward to the 

 lachrymal ; auditory bullae moderate ; paroccipital processes long ; 

 bony palate perfect, produced behind the molars ; mandible resembling 

 in form that of Leporidce. Clavicles perfect ; scapular and humerus 

 resembling those of Castoridce. Both fore and hind limbs with five 

 digits, claws probably short and hoof-like. Ischia articulated with two 

 of the caudal vertebrae. Pliocene of South America. Fossil genus : — 



I. Mesotherium, Series, Compt. Rend. Ac. Paris, xliv. p. 961 



(1857). 

 (Characters those of the family.) 



P.S. Since this paper was written I have been much pleased to 

 find that the researches of my friend Mr. A. Doran, of the Royal 

 College of Surgeons, into the form of the auditory ossicles of mam- 

 mals, essentially confirm the views of affinity here adopted. Mr. 

 Doran' s observations are yet unpublished; and it will be sufficient 

 here to indicate their more general result. He finds in the auditory 

 ossicles of the more typical Sciuromorpha a different type from that 

 presented in the typical Myomorpha. Castor, though aberrant, 

 approaches nearest to Arctomys ; and Anomalurus is very close to 

 Sciurus. Among the Myomorpha, Bathyergus has the characteristic 

 type of the Hystricomorpha, in which, with few or no exceptions, the 

 malleus and incus are ankylosed together ; and Dipus inclines in the 

 same direction. The ossicles of the Glires duplicidentati pre- 

 sent a distinct and less specialized type. 



