DAPLODONTIDiE— SKULL OP HAPLODON ItUFUS. 



565 



include many details, deinonstratiog the really cluso relationships of the ani- 

 mal to the Sciurida. The interorbital foramen (u8iially a considerable index 

 of affinity in Rod^ntia), however, is little like thotof Sciuromorphs generally, 

 v\ nile it is not at all like that typical of Myomorphs. 



Viewed from above, without taking the rostrum into consideration, tiie 

 general contour of the skull represents a nearly equilateral triangle with 

 rounded-ofT |K>8tero-lateral angles, somewhat convex sides, and sinuous base. 

 There is a very deep notch or cmargination just behind the zygomata, some- 

 times almost converted into a foramen by apposition of the posterior angle of 

 the zygoma against the wall of the produced auditory bulla. Within this 

 triangular contour are a pair of triangular vacuities — the conjoined orbital and 

 temporal fossa;. The posterior extremities of the zygomata arc much further 

 apart than a distance equal to their extreme length. The rostral portion of 

 the skull forms rather more than a fourth of the total length ; the rostrum is 

 stout, its width being contained only about 4^ times in the total length ; it is 

 approximately cyliDdrica), with nearly parallel sides converging anteriorly, 

 and tumid nasal portion. The nasal bones are truncate anteriorly, and do not 

 project beyond the plane of the incisive alveoli. From the base of the ros- 

 trum on each side^ the zygomata suddenly stand oiT, overshadowing the ante- 

 orbital foramina, which are therefore not visible in this view. Thence the 

 zygomata curve gently outward and backward, then rather suddenly contract, 

 the point at which they are farthest apart being but little in advance of their 

 posterior ends. Behind their ends is the deep emargination, as seen also in 

 Arctomys, Castor, and other Sciuromorphs, but in this case narrower for its 

 depth, appearing almost like a fissure. This separates the zygomatic arch 

 from the prominent angular postero-lateral corner of the skull, formed by the 

 mastoid and auditory bullae. The posterior outline, on the whole exactly 

 transverse, is sinuous, like the human lip or "Cupid's bow''; for the promi- 

 nent sharp occipital ridge which defines the occipital plane from that on top 

 the skull, curves backward at first from the corner of the skull and then more 

 strongly forward to the median line, this latter anterior convexity sufficing to 

 throw some of the occipital surface into view from above. The roof of the 

 brain-case is strongly convex and sloping ou the sides, though little arched 

 longitudinally ; it then spreads more horizontally to form the occipital flanges. 

 This surface is marked lengthwise with a pair of raised lines, indicating the 

 boundaries of the temporal foesse ; at the back border of the orbit, just within 



