664 



RODENTIA 



distribution or texture, its quality somewhat harsh and loose. 

 Mamrn* : p 2-2, « 2-2 = 8. 



SA-ull. — The skull is moderately broad and rather deep, attain- 

 ing a marked degree of angularity in old age. Dorsal profile 

 moderately convex throughout, the interorbital region usually 

 fiattish, the nasals sloping forward at an angle of about 18°. 

 Occiput sufficiently oblique for the condyles to be visible when 

 skull is viewed from above. Ventral profile with no special 

 peculiarities ; contrast between occipital depth and that of rostrum 

 very pronounced. Brain-case oblong, well squared anteriorly by 

 the prominent postorbital processes, its lateral borders nearly 

 parallel except for the break caused by the zygomatic roots ; 

 outer edges of jjarietals marked in old age by well developed 

 longitudinal ridges which are continuous po.steriorly with lambdal 

 crest and which come together abruptly in front to join the 



interorbital ridge. No trace of 

 sagittal crest ; lambdal crest repre- 

 sented by the outer extremities 

 only. Interparietal large, its area 

 nearly equal to that of parietal, 

 its form somewhat variable, but 

 posterior border usually straight 

 or nearly so, anterior border bicon- 

 vex with median projection, the 

 outer extremities rather abruptly 

 truncate : greatest antero-posterior 

 diameter, exclusive of median pro- 

 jection sensibly less than half 

 transverse diameter. Depth of 

 occiput when viewed from behind 

 more than half width, the brain- 

 case rising slightly above occipital 

 level ; paroccipital processes well 

 developed, their tips extending 

 noticeably beyond level of lower lip of foramen magnum, their 

 bases continued upward as conspicuous ridges along suture 

 between occipital and squamosal. Floor of brain-case without 

 special features, the basioccijjital with ill-defined median ridge ; 

 width of basioccipital along anterior suture contained about 

 three times in median length. Auditory bulla; large, evenly 

 inflated, without much spongy tissue, the surface smooth and 

 shining ; beak moderately developed, closely applied to outer side 

 of base of pterygoid ; region near meatus slightly flattened. 

 Interorbital region narrow, sub-cylindrical, with distinct temporal 

 ridges which eventually unite to form a definite median crest 

 extending well forward to nasals. Z3-gomata wide spreading 

 and short, a small portion near middle essentially parallel with 

 main axis of skull ; median expansion evident but not very wide ; 

 anteorbital foramen narrow and high, the plate forming outer 



I'lfJ. 129. 

 Microtus agrestis. Nat. size. 



