348 E. L. Troxell — Palceolagus, an Extinct Hare. 



tillus. Note the very deep indentations in P3, the wide 

 separations of the lobes of each P4M1.0 and also the 

 antero-posterior extension of M3. 



The very young individual illustrated in figure 20 not 

 only shows the two milk teeth well preserved, but also the 

 anterior half of the first molar pushing up just behind. 

 This portion of molar in its unbroken enamel demon- 

 strates the fact that the two parts of the tooth are simply 

 cemented together, having no intimate connection. 



Interpretation of milk dentition. — The question arises 

 often as to what relation the milk teeth bear to the perma- 

 nent teeth both (1) of the time, (2) of the descendants, 

 and (3) of the ancestors. The three lobes of the first 

 deciduous lower tooth seem to be prophetic of Lepiis, 

 the modern representative of the race ; sometimes the 

 second deciduous tooth shows the features of the tooth to 

 follow in its place, having the small posterior cone in 

 addition to the two main lobes ; but sometimes the teeth 

 are patterned in retrospect. 



Probably the most interesting comparison may be made 

 between Mj of specimen No. 12076 (fig. 17), with its many 

 cusps, and the permanent short-crowned molar of Ictops. 

 The arrangement of the cusps of the talonid or posterior 

 lobe, the transverse arrangement of the anterior half of 

 the tooth, and the low area between the lobes, in Palceo- 

 lagus having three tubercles, seem to link this specimen 

 up with the insectivores. Of course the teeth of the latter 

 are much more primitive in their brachyodonty, and they 

 differ in having an additional cusp in front of the large 

 double-cusped anterior lobe. 



Summary. 



It is difficult to draw the dividing line between the 

 Oligocene species of Palceolagus except for P. haydeni 

 and P. turgidus. P. agapetillus and P. intermedins are 

 apparently distinguishable from P. haydeni on the basis 

 of size alone, and are here considered as subspecies. 

 Although there is a very great resemblance between the 

 Oligocene forms and the Recent ones, yet there are strik- 

 ing differences, and most of those of the skeleton may be 

 ascribed to a cursorial adaptation in the Recent form, 

 Lepus; in this connection the position and size of the 

 nares are of especial importance. 



A number of views of the teeth are given to illustrate 

 variations, mostly due to age, and to publish for the first 

 time detailed drawings of the milk dentition. 



