470 DK. C. I. rOllSTTH MAJOE ON 



Leporidas from the Lower Miocene (or it may be from the Oligocene) to the present 

 time. 



The incisors provided with enamel-folds point back towards cuspidate incisors, for 

 the enamel-folds of lophodout and laminated teeth are obviously the derivates and 

 horaologues of the " valleys " separating the cusps or tubercles. Now it is very 

 suggestive that we meet with cuspidate incisors in Plesiadapis, a genus from the lowest 

 Eocene of Rheims, classed among the Lemuroidea by Lemoine and other writers, 

 considered by Schlosser and me to be a very primitive Rodent. In the jaws of 

 Plesiadapis the teeth are greatly reduced in number. In the lower jaw we have only 

 one powerful elongated incisor, directed obliquely forward and upward, and separated 

 from the five cheek-teeth — the premolars being already reduced to two — by a con- 

 siderable diastema. On its posterior face the lower incisor has a cingulum supporting a 

 small cusp. The upper incisors, too, are separated by a long interval from the five 

 cheek-teeth, and appear to have been three in number (Lemoine considers the very small 

 outer one to be the canine). The two outer pairs are very small and unicuspidate ; 

 the inner pair robust, generally tricuspidate, there being an anterior pair of cusps, and 

 backwardly an additional cusp, which starts from a kind of cingulum *. 



If we imagine the cusps of these upper incisors of Plesiadapis to have become 

 lengthened in accordance with a general change of the more brachyodont incisors 

 into a hypselodont one, and their interstices filled with cement, so that by trituration 

 a level surface can be produced, the result would be a pattern somewhat similar to 

 that of several of the figured Leporida^. The posterior cusp of Plesiadapis, projecting 

 from behind into the cavity f , would produce a posterior ramification like that of the 

 Leporidse. 



The test will lie in the search for Tertiaiw Leporidse exhibiting an intermediate stage 

 between the condition of the upper incisors of Plesiadapis and that of recent Leporidge. 

 An examination of the incisors of Paheolagtis might decide the question. 



Genus Palji;olagus. 



Palceolagus, from the Tertiary of North America, is represented by Leidy | and by 

 Cope § as showing in the teeth only one character distinctive from the genus Lepus, 

 viz. the more simple conformation of the anterior inferior premolar of the extinct genus, 

 and of this character more hereafter. When, however, we go over the descriptions, 

 accompanied by numerous figures, and an examination of originals, several of which are 

 in the British Museum, we cannot but be struck at once by some very essential differ- 

 ences in the triturating surfaces of the two genera. When do we ever meet with 

 molars in any species of Lepus showing the complete absence of all traces ^of 

 enamel, with the exception of part of the marginal border % This is the case in old 



* Lemoine, in Bull. Soc. Gcol. France, xix. 1, p. 278, pi. x. iig. ."^O, n, /;, t (1891). 

 t Lemoine, 1. c. pi. x. fig. 50, 6, c. 



+ Proc. Acad. Philadelphia, p. 89 (1850) ; id. ' Extinct Mammalia of Dakota and Nebraska," p. 332, pi. xsvi. 

 figs. 14-20 (1869). 



§ ' The Vertebrata of the Tertiary Formations of the West,' i. p. 870, pis. Ixvi., Ixvii. (1883). 



