E. L. Troxell—Palceolagus, an Extinct Hare. 343 



The fibula is fused to the shaft of the tibia in its distal 

 half in a manner identical in both forms, showing the 

 already progressive character in the Oligocene, or else 

 the conservatism of the Recent, or both. The blade of 

 the ilium is very straight and narrow m Palceolagus, 

 especially at the sacral suture. The scar of this suture 

 is V-shaped, and longer than wide. In the Recent form, 

 this suture is U-shaped, wider than long, and beyond it 

 there is a roughened area for tendinous attachment. 



Concerning the loss of the coronoid^ in later genera- 

 tions, it is apparent that along with the deepening of the 

 jaw and the lengthening of the ramus, the temporal 

 insertion on this process becomes weaker and weaker until 

 now the anterior border of the ascending ramus, a double 

 edge presenting a broad grooved surface, is reduced to a 

 straight line. Furthermore, it is suggested that the 

 lateral movement of the rabbit's jaw is brought about by 



Fig. 4:.— Palceolagus turgidus. Cat. No. 10306, Y, P. M. Crown view of 

 upper molars and premolars, P^ lost. X 2- 



muscles attached to the angle and to places other than on 

 the coronoid process, and that the broad incisors of the 

 modern Lepus may also be correlated with an orthal 

 movement of the jaws. 



Matthew has pointed out that the head in Palceolagus 

 is carried low, with the nose extended forward, as a result 

 of the small basicranial angle ; this is in contrast to Le- 

 pus, where the head is bent more strongly downward, 

 shomng, he says, an adaptation to a running habit. In 

 harmony with this it is interesting to note that the large 

 posterior nares of Lepus (fig. 3) and their position far 

 forward, mentioned in the table of characters, are appar- 

 ently a readjustment to facilitate breathing by shortening 

 and enlarging the air passage. And so the whole struc- 

 ture of the Recent genus : the form of the femur, the 



" The presence or absence of this process is not entirely uniform in the 

 Recent hares. L. americanus, the so-called varying hare, has a very thin 

 bone extending forward from the antero-external edge of the ascending 

 ramus, which does not appear in certain rabbits. This may be a vestige of 

 the coronoid process. 



Am. Jour. Sci. — Fifth Series, Vol. I, No. 4.— April, 1921. 

 23 



