SKELETON IN THE FLYING LEMURS. 151 



Passing to one of the third lower incisors a very different tooth is 

 seen. The coronal portion is antero-posterioi'ly elongated: as naturally 

 implanted, it is directed upward; it is slightly concave on buccal aspect 

 and correspondingly convex on its outer surface; it presents five pectina- 

 tions which become smaller and smaller from before backward; the 

 fifth pectination is sometimes very rudimentary. This tooth is much 

 thicker than any one of the four thin, front incisors just described. - 

 These six incisors are very nearly, or quite, in contact and there is no 

 tnie diastema among the teeth of either jaw. The veiy wide interval 

 l)etween the two anterior incisors of the upper jaw can hardly he con- 

 sidered a tnie diastema. 



The last premolar, and the true molars of either side of the mandible, 

 develop from four to five sharp-pointed, trihedral cusps that vary con- 

 siderably in size and, packed together along the alveolar border, lend to 

 the series a very complicated appearance. The largest cusps are external 

 and median, while the much smaller ones are, as a rule, ranged internally. 



With the jaws normally articulated and tightly closed, the abruptly 

 inturned, sharp-pointed, external cusps of the molars and last premolars 

 of the upper jaw lie entirely without the alveolar margin of the lower 

 jaw, and in mastication act as true cutters rather than as grinders. When 

 the mandible is thus normally articulated, and we regard the slaill as a 

 whole upon its basal aspect, all these aforesaid cusps are in full view, 

 even tlieir apices, which is a very remarkable arrangement. 



Beturnrng to the base of the skull we next observe the far backward 

 extension of the pterygoidal wings of the sphenoid, with the rather deep 

 longitudinal valley that exists between them. These wings turn slightly 

 outward, and at the postero-inferior angle of each there is to be noted 

 the not very lai'ge, bifurcated, hamular process already referred to above. 



Passing mesially from the posterior end of the vomer from beneath 

 the posterior nasal sj)ine, there is a prominent ridge, or line, running 

 backward almost to the anterior margin of the foramen magnum. This 

 line is most conspicuous and sharpest in its anterior part, and at its 

 posterior ending merges gradually into the general surface on the basi- 

 temporal. Between the backward sloping termination of a pterygoidal 

 wing and the glenoid fossa, upon either side, occurs the foramen ovale, 

 the largest pair of foramina in the basis cranii. Each transmits, on its 

 own side, the third division of the fifth nerve, the small meningeal artery, 

 and the small petrosal nerve. 



The glenoid fossaj constitute prominent features on the base of this 

 skull; either one is large (elongo-elliptical in outline, with the major 

 axis perpendicular to the cranial axis), flat, and smooth, the articular 

 surface being in the horizontal plane, and comj)letely overlying the inferior 

 surface of the posterior root of the zygoma. (Plate I, figure 1.) These 

 fossa? are separated mesially by an average distance of 15 millimeters. 



