In the ancestral lineage, the ridge is distinct but relatively low; the masseteric fossa is shallow and is a poorly developed area for attachment of the superficial masseter muscle. In modern Geomyinae the ridge is massive and forms a high crest, especially anteriorly, and the masseteric fossa is a deep, prominent cup along the dorsal side of the crest. The elaboration of the crest and fossa evidently is associated with an increase in size of the superficial masseter muscle, which enlarges and provides increased power for the propalinal type of mastication. A high crest has evolved independently in both modern lineages, Thomomyini and Geomyini. _Basitemporal Fossa_ The name basitemporal fossa is suggested here to denote the deep pit that lies between the lingual base of the coronoid process and the third lower molar. The basitemporal fossa receives the insertion of the temporal muscle. The fossa, which until now has not been named, is a unique feature in advanced Geomyinae, being unknown in either primitive Geomyinae or in other rodents. The temporal is one of several muscles holding the occlusal surface of the lower molariform dentition firmly against the upper cheek teeth during mastication. In primitive geomyines that masticate food by a planing action, the temporal muscle also moves the mandible posteriorly and food is ground between the enamel plates when the lower jaw is retracted as well as when it is moved forward. The basitemporal fossa appears in late Pliocene geomyines and increases the attachment surface of the temporal muscles that powers the planing action important in utilizing woody and fibrous foods. The basitemporal fossa developed in only one of the modern lineages (tribe Geomyini), the same lineage in which grooved incisors evolved. Both features probably are adaptations for feeding on coarse food. The fossa is not greatly developed in either the ancestral tribe Dikkomyini or the modern tribe Thomomyini, although in some specimens a slight depression marks the site of the basitemporal fossa. [Illustration: FIG. 1. Types of skulls in the subfamily Geomyinae. × 1. A. and B. Generalized type of skull. _Geomys bursarius lutescens_, adult, male, No. 77955 KU, 10 mi. N Springview, Keya Paha Co., Nebraska. A. Dorsal view of skull. B. Ventral view of lower jaw. C. and D. Dolichocephalic type of skull. _Orthogeomys_ (_Orthogeomys_) _grandis guerrerensis_, adult, female, No. 39807 KU, 1/2 mi. E La Mira, 300 ft., Michoacán, México. C. Dorsal view of skull. D. Ventral view of lower jaw. E. and F. Platycephalic type of skull. _Pappogeomys_ (_Cratogeomys_) _gymnurus tellus_, adult, female, No. 33454 KU, 3 mi. W Tala, 4300 ft., Jalisco, México. E. Dorsal view of skull. F. Ventral view of lower jaw. ]