26 MICROTIN^ 



Parsons ^ describes and figures the " parietal portion " of the 

 temporalis in the Water Vole as being continuous with what is 

 described above as the anterior portion of masseter medialis. The 

 foregoing description is based chiefly upon several dissections of the 

 jaw muscles in the same species ; I have never found any portion 

 of the temporalis going to an insertion external to the coronoid 

 process (apart from the fibres that envelop the tip of the 

 process) and my experience in this respect seems to agree with 

 that of Tullberg.2 



Pterygoid internus. This is a short, thick muscle passing 

 obliquely backwards, outwards, and downwards from its origin 

 in the pterygoid fossa to its insertion upon the inner face of the 

 mandibular angular process. 



Pterygoid externus. A small muscle arising from the 

 ectopterygoid plate and inserted upon the inner face of the condylar 

 process of the mandible. 



DiGASTRicus. This muscle arises from the paroccipital process 

 and is inserted into the lower border of the mandible towards the 

 symphysis. Anteriorly its belly is in close contact with that of its 

 fellow of the opposite side. Its function is to retract and depress 

 the mandible. 



d. The Evolution and Status of the Subfamily Microtin^, 

 AND THE Interrelationships of the Genera. 



There can be no doubt that the Microtinse have descended 

 from a primitive Myomorphous stock which was also ancestral 

 to all the other groups of Muridse. Among Muridse the sub- 

 family is sharply defined by its cranial and dental characters. 

 Of these the most prominent are the firm construction of the skull, 

 shortened rostrum, forwardly placed orbits, peculiarly formed 

 " masseteric " plates, the presence of post-orbital squamosal 

 crests or processes, the thickened palatal processes of the maxil- 

 laries and palatines, and the hypsodont prismatic cheek-teeth. 

 The molars of certain Cricetinse {e.g., the North American genera 

 Sigmodon and Neotoma) show a strong superficial resemblance to 

 the cheek-teeth of Microtinae; but detailed study shows that 

 similar tooth-patterns have been evolved in somewhat different 

 ways in the two subfamilies. In the remarkable Asiatic genus 

 Myospalax (" Siphneus ") the cheek-teeth are rootless and closely 

 resemble in pattern those of the typical lemmings among Micro- 

 tinse. But although the skull (Figs. 15-17) is highly speciaHzed 

 for fossorial habits, it and the jaw muscles resemble those of the 

 Cricetinse in retaining essential features similar to those found in 

 the more primitive of the non-Microtine Muridse generally, and 

 differing widely from those characteristic of the Microtinse. 

 Among the most primitive Nesomyinse (e.g., Nesomys), now 



1 Paesons, P.Z.S., 1896, p. 160. 



2 TuLLBEEG, Ueber d. System der Nagetiere, Taf. xiv, figs. 17 and 20. 



