370 U. S. P. R. R. EXP. AND SURVEYS ZOOLOGY GENERAL REPORT. 



distinct from the squamous. They are of considerable extent, entering into the occipital sur 

 face, and very largely into the under part of the cranium. The meatus externus is broad and 

 tubular, with thin walls, its axis projecting forwards, outwards, and upwards ; and it is 

 embraced posteriorly by a thickened process of the squamous bone. 



The malar bone is very short ; its posterior half is concealed from view from above by the 

 process of the temporal ; anteriorly it is more or less enlarged, and fits into a notch of the 

 zygomatic plate of the maxillary, which forms the whole anterior portion of the zygoma, and 

 sends a thin plate backwards under the malar. Thus, in consequence of the forward projection 

 of the temporal on the upper surface of the zygoma, and the backward projection of the maxil 

 lary on the under, the two points leave but a slight interval between them when the malar bone 

 is removed. 



The bony palate of Geomys is much restricted, being really confined to a narrow space between 

 the molars. This is convergent anteriorly, where it is not wider than the molars ; it is traversed 

 by two deep grooves, leaving a central and two lateral ridges of nearly equal height ; the central 

 one is prolonged anteriorly to the incisive foramina. A plane tangent to the surface just described 

 would show a considerable interval between it and the cranium or petrous bones behind, and a 

 still greater one in front, the outline of the under surface of the bone rising rapidly from the 

 molars to near the incisors, and then curving a little down again. The palatine surface here is 

 narrow, the cross section being truncate wedge-shaped, the outline widening above. The inter 

 maxillary suture is seen on the side of the muzzle, about its middle ; just within it is seen a deep 

 depression, at the bottom of which is the anterior termination of the ante-orbital foramen ; this, 

 however, does not open through the bone into the nasal cavity, as in Dipodomys. The incisive 

 foramina are entirely in the intermaxillaries. The zygomatic plate of the maxillary is broad and 

 oblique, standing out in a plane perpendicular to the axis of the skull (the two of opposite sides 

 being nearly in the same plane) ; it is nearly plane anteriorly; its lower margin rises rapidly from 

 the alveoli of the molars to a point opposite the centre of the skull, and then curves somewhat 

 downward again ; it is thickened superiorly with a flattened surface, which expands behind to 

 receive in its notch the end of the malar. 



It has already been stated that the lower jaw is very massive ; much more so than in any 

 other American rodents. There is scarcely any postero-inferior angle, this being represented by 

 a compressed ridge posteriorly, commencing below the root of the last molar. The upper por 

 tion of this ridge turns off horizontally, however, and is continuous with a horizontal thickened 

 process standing out perpendicularly to the side of the jaw ; this represents the upper corner 

 of the postero-inferior angle. The lower outline of the jaw is a nearly equable and convex 

 curve throughout, mostly parallel to the inferior surface of the incisor. The condyloid has 

 scarcely any neck, although rather a deep notch separates it from the coronoid process, which 

 rises considerably above the level of the condyle. There is a very deep pit below the base 

 of the coronoid process, and exterior to the last molar. The covering of the posterior extremity 

 of the lower incisor is seen as a very prominent tubercle (larger than the condyle) which is 

 situated on the outside of the jaw, and at a height about midway between the condyle and the 

 horizontal process already described. There is also an excavation or pit running along the out 

 side of the jaw, internal and anterior to this tubercle. 



The incisors, as already remarked, are of extraordinary strength and size ; more or less plane 

 anteriorly ; the upper grooved, the lower perfectly smooth. A section of the teeth is wedge- 

 shaped ; the base anterior. The groove in the upper incisor is either central, or central and a 



