MYOMOKPHA. 857 



space. There is no postsquamosal foramen in the recent genera. In Dipo- 

 domijs tlie 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 alveolus forms a convexity on the outer side below the coronoid 

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

 tunied outwards at the apex. The degree of obliquity of the grinding 

 surfaces of the molar teeth vaiy 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 which apparently 

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

 was evidently a well-developed tail. The scapiila 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 off as to be little promi- 

 nent. 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 pos- 

 teriorly at the base. The femur is not elongate. Its trochanters are well- 

 marked, including a third. This is wanting in Thomomys hulbworiis. 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 directed outwards, but does not project much at 

 the head. 



Individuals of this genus were very abundant in Oregon during the 

 middle 3Iiocene 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 modifications in the 

 forms of the premolar teeth, diff'erences in the length and width of the 

 muzzle, and some range in dimensions. 



