No. 2.] COPE ON MIOCENE KODENTIA. 379 



ferent is seen in Neotoma, Hesperomys^ &c. Snpraoccipital distinct on 

 superior face of skull. Paroccipital process small or none. Mastoid 

 elongate, adherent to otic tube. No postfrontal process. 



A well-marked character which distinguishes the skull of this genus 

 from Thomomys, Bipodomys, &c., is the separation of the meatal tube 

 of the otic bulla from the zygomatic process of the squamosal bone by 

 an interspace. There is no postsquamosal foramen in the recent genera. 

 In Dipodomys the otic bulla is more largely developed, but it has the 

 anterior bottle-neck prolongation seen in Entoptychus. 



In the mandible the coronoid process is developed, but is not large 

 It is well anterior to the condyle, which it somewhat exceeds in height 

 The incisive aveolus forms a convexity on the outer side below the coro- 

 uoid process. The angle is prominent, and is at first incurved below, 

 and then turned outwards at the apex. The degree of obliquity of the 

 grinding surfaces of the molar teeth vary with the species. 



Parts of several skeletons are in my collection, but I cannot attach 

 them to any cranium. They present the general characters of the genus 

 Thomomys so far as they go. I describe some bones Avhich apparently 

 belong to one individual. The sacral vertebrae carry neural spines. 

 There was evidently a well-developed tail. The scapula has a narrow 

 glenoid cavity ending in a tuberosity adjacent to the coracoid hook. 

 The spine is robust, terminating in a stout acromion. The tuberosities 

 of the humerus are situated below the head, and are so rounded oft' as 

 to be little prominent. One side of the greater is continued into a very 

 prominent deltoid crest, which terminates abruptly below. The ilium 

 has a narrow trilateral neck, and a projecting anterior inferior spine. 

 The pubis is directed posteriorly at the base. The femur is not elon- 

 gate. Its trochanters are well-marked, including a third. This is want- 

 ing in Thomomys hulbivorus. The neck rises obliquely to the rather large 

 head. The condyles are short and spreading, and the rotular groove is 

 short and rather wide, and with well-marked ridges. The tibia is much 

 curved backwards at the proximal part. The crest is acute and is di- 

 rected outwards, but does not project much at the head. 



Individuals of this genus were very abundant in Oregon during the 

 middle Miocene epoch. They represent several species, but how many it is 

 difficult to determine. The most noteworthy variations are found in the 

 development of superciliary ridges; then there are modiiications in the 

 forms of the premolar teeth, differences in the length and width of the 

 muzzle, and some range in dimensions. 



The followiug table represents the characters of the species so far as I 

 can determine them at present : 



A. Thickened superciliary ridges waiithig; front wide. 



Superciliarj- liorders obtnse, not coutinned into temporal ridges ; front flat, or 

 or little concave ; premolars narrow. 



Length of skull .046 E. planifro)i8. 



Length of skull .038 .E. minor. 



