16 MICROTIN^ 



importance to the systematist. In some genera the palate 

 terminates behind in a simple transverse shelf with or without 

 a median process or " nasal spine " ; in these the postero-lateral 

 pits lie at a deeper level and pass forwards under (dorsal to) the 

 palatal shelf, and their inner borders do not form any externally 

 visible connection with the shelf or with its " nasal spine " (e.g., Evo- 

 tomys, Dicrostonyx, Lemmus). In other genera the " nasal spine " is 

 elongated, is inclined dorsally instead of being horizontal, and is 

 connected at its tip and sides with the inner borders of the postero- 

 lateral pits, thus becoming converted into a sloping median 

 septum between the pits. In the more primitive of these genera 

 the median septum is short, broad, and ill defined, but in the more 

 specialized ones it is long, narrow, and very sharply defined ; 

 frequently the ventral surface of the septum is grooved, and some- 

 times it is completely cleft. The floors of the postero-lateral pits 

 are usually perforated by numerous small foramina which serve 

 for the transmission of nerves and vessels to the soft palate and 

 upper part of the pharynx ; but in Prometheoinys, in which the 

 hinder part of the palate is generally speaking somewhat inter- 

 mediate between the two types described above, the floor of each 

 pit is occupied by a single very large foramen. 



The pterygoid fossae vary in size and depth from genus to genus, 

 but the ectopterygoid plates are always well developed. When 

 deepest the floors of the fossse lie at a level distinctly dorsal to 

 the ventral surface of the basisphenoid and by means of a small 

 transverse canal perforating the latter they are placed in com- 

 munication with each other. To a large extent the size of the 

 pterygoid fossae, the width of the choante, of the back part of the 

 basisphenoid and of the fore-part of the basioccipital seem to be 

 dependent upon the varying size of the auditory bullae. 



Some of the most characteristic features of the Microtine skull 

 are shown in longitudinal vertical sections (Figs. 9 and 10). In 

 other Muridae the palatine processes of the maxillary and palatine 

 bones are thin ; but in Microtinae they are enormously thickened, 

 probably in correlation with the unusually tall and robust crowns 

 of the molars and with the powerful development of the jaw 

 muscles. The hinder part of each palatine, i.e., the portion 

 which forms the post-palatal pit, remains thin. The presphenoid, 

 the basisphenoid and the fore-part of the basioccipital are con- 

 siderably thickened vertically. In the floor of the braincase 

 (Fig. 11) the presphenoid is of unusual length, in correlation with 

 the great length of the tooth-rows ; but owing to the intrusion of 

 the alveolar capsules of m^ and m^ into the sphenorbital fissure, 

 this bone and the fore-part of the basisphenoid are greatly com- 

 pressed laterally. The sphenorbital fissure occupies a great area 

 in the cranial floor, but in the more hypsodont genera its outlets 

 are greatly straitened by the tooth capsules, which divide the 

 fissure into two parts, an inner and an outer ; the inner division 

 between the tooth capsules and the presphenoid transmits the 



