46 The Extinct Rodentia of North America. (January, | 
crests are obsolete or wanting. The positions corresponding to : 
their external extremities are marked by more or less distinct 
cusps. There is a single internal tubercle of the crown. In the 
third and fourth molar of P. delcatissimus I observe rudiments of — 
a second internal tubercle. The incisor teeth are compressed, — 
with narrow anterior face. The enamel is not grooved, and is — 
little or not at all inflected on the inner side of the shaft, while it l 
is entirely so on the external face. 4 
There is a large round foramen infraorbitale exterius, like that 
of Ischyromys and Fiber, and entirely unlike that of Gymnopty- _ 
chus and Sciurus, conforming in this respect to the forms of the ; 
extinct group of the Protomyidz of Pomel. 
The cast of the brain indicates smooth oval hemispheres, which 
leave the cerebellum and olfactory lobes entirely exposed. The 
latter are ovoid and expand 
ed laterally. 
The species from whicli 
most of the characters of the 
genus as above stated have 
been derived are the P. deli- 
catior and P. delicatissimus. 
They further display the fol- 
lowing general characters: 
The anterior limbs are rela- 
| tively longer than in recent 
species of squirrels. 
a 
Ji 
i; 
a 
= 
= 
= 
posteriorlimbislarger. These — 
points indicate approximation — 
to the cotemporary Me 
_ don rs. 
—Bones of the specimens of Plesi- sn ne cement ) 
altas delicatissimus Leidy, represented in No characters have 
Fig. 1. Fig. a, humerus, front view ; 4, prox- i dis 
mal e of ulna and radius. Fig. ¢ 5 Risti eee rs Tey MoT g i 
cart oF tibia posterior side; d, same Pad be- tinguish the American 
low; e, astragalus from above ; , astragalus „nu! 
and calcaneum, distal ends. ciel trata cies as representing a gen 
Vol. Iv, Report U. S. Geol. Survey Terrs. distinct from the Plesiar 
Pie. 2. 
