78 MICROTIN^ 



hand; there are five plantar tubercles; the sole is naked 

 between the pads, but is densely haired posteriorly. The tail is 

 about half the length of the head and body, thick, and densely 

 clothed with hair. The mammary formula is 2 — 2 = 8. 



In general form the skull is much as in other voles, with 

 moderately long and broad rostrum, rather sharply constricted 

 interorbital region, squarish braincase, normal zygomata, and 

 with the infraorbital canal and its outer wall of the usual form. 

 The rostrum is not unusually shallow; the occiput is vertically 

 truncated. One primitive feature of the dorsal surface is worthy 

 of special remark ; traces of the sagittal suture can be seen, even 

 in old adults, between the nasals, frontals and parietals, and 

 posteriorly the suture usually persists, dividing the remarkably 

 small interparietal into two halves. In adults the temporal ridges 

 are fused to form a sagittal crest which ultimately extends back- 

 wards from the middle of the interorbital region to the occiput ; this 

 crest reaches its greatest height on the posterior part of the frontals, 

 and in all but the very oldest specimens the crest subsides in the 

 mid-parietal region and the ridges, thence feebly marked, diverge 

 slowly to the occiput, crossing the interparietal obliquely ; the inter- 

 parietal, at all times small in Prometheomys, diminishes with age. 

 The squamosals are very large, forming the whole dorso-lateral 

 surface of the braincase, their upper borders being parallel; 

 the lateral wings of the parietals, so constantly present in voles, 

 are obsolete; the post-orbital crests, though fairly extensive, 

 are very weak, and the squamosals anteriorly show no special 

 tendency to approach each other by encroaching upon the frontals. 

 These characters are correlated with the great development of 

 the temporal muscles, of which the middle and hinder portions 

 are especially strong ; a parallel development is seen in Ellobius, 

 and a somewhat similar, but, for Microtinse, far less abnormal 

 condition occurs in Ondatra. The alveolar capsules of the cheek- 

 teeth are, in young specimens, protuberant in the floors of the orbit 

 and the sphenorbital fissure, but these protuberances subside in 

 adults as the crown stumps are pushed out of the capsules by the 

 more slender roots; the capsule of m^ is nearly clear of the 

 braincase. The antero-palatal foramina are moderately large. 

 The palate is abnormal behind, more primitive than in any other 

 member of the subfamily, but foreshadowing the palate of 

 Ellobius, etc. ; it is sculptured in low relief, with broad and 

 complete postero-lateral bridges, but with the median septum 

 represented only by a broad irregular nasal spine ; the floors of 

 the homologues of the postero-lateral pits lie slightly dorsal to 

 the general surface of the palate, but they are cut off from it by 

 the lateral extensions of the fore-part of the mesopterygoid fossa ; 

 each pit is formed as usual by the ventral surface of the palatine 

 bone, but each is perforated by a large foramen which occupies 

 fully half the area of the pit. By filling up the foramina, deepen- 

 ing the pits, placing their edges in continuation with the nasal 



