176 THE WASATCn AND BEIDGER FAUN^. 



wanting:. The positions coiresponding to their extern.al extremities are 

 marked by more or less distinct cusps. There is a single internal tubercle 

 of the crown. In the third and fourth molars of P. deUcatissimns I observe 

 rudiments of a second internal tubercle. 



The incisor teeth are compressed, with naiTOw anterior face. The 

 enamel is not grooved, and is little or not at all inflected on the inner side 

 of the shaft, while it is extensively so on the external face. There is a large, 

 round foramen infraorhitale exterins, like that of Ischyromys and Fiber, and 

 entirely unlike that of Gymnoptychus and Sciurus, conforming in this respect 

 to the forms of the extinct group of the Protoniyides of Pomel. 



The cranium of the specimen originally described by me as Pseudoto- 

 mus hians, exhibits the following characters: The superciliary margin is 

 short, and without jiost-frontal process. The temporal fossa? are large, and 

 contract the brain case behind the orbits to a striking degree. Their ante- 

 rior margins rise from the post-frontal angles and converge backwards, 

 meeting in a sagittal ridge opposite the anterior part of the squamosal bone. 

 The parietal bones increase rapidly in width to the squamosal, which are 

 largely inferior at their zygomatic portion. They do not extend very far on 

 the superior aspect of the skull, nor backwards beyond the auditory meatus. 

 The occipital region is concave, and sun-ounded by a prominent crest 



The foramen infraorbilate exterius has an inferior position, being a little 

 above the alveolar border. Tliere is a prominent tuberosity on the under 

 side of the basal front of the malar bone, just exterior to the position of the 

 second molar of Ardomys ; its inferior face is truncate. 



The pterygoid laminaj are jjrolonged, inclosing a trough. Their 

 sphenoid ala? descend steeply from their posterior base, and have an external 

 ridge, which marks out a pterygoid fossa. The otic bulla is not large. 

 Paroccipital process distinct. T\\q foramen ovale is large, and is divided by 

 a thin bridge of bone. The two external foramina resulting are also the 

 alisphenoids. There are no additional foramina in this region. The space 

 for the otic bulla is moderately large; the basicranial axis is grooved at 

 the junction of the basioccipital and sphenoid bones. The zygomatic arch 

 is deep and thin. The glenoid cavity is wide but longitudinal. 



The cast of the brain indicates smooth oval hemispheres, which leave 



