FOSSIL AND EECENT LAGOMORPIIA. 477 



of wear of the deciduous tooth of Frolagus, the whole of the anterior tripartite lobe 

 appears invariably connected with the posterior part of the tooth by a dentinal isthmus, 

 thus giving the whole tooth some resemblance to m. 1 inf. of a vole ; and it has, in 

 fact, been mistaken for a molar of Mtcrotus. 



A characteristic feature of the anterior lower premolar, '^7^, of Prolafjus, is an odd 

 isolated cusp or pillar, connected only by cement with the rest of the tooth, and 

 situated on its anterior side, thus giving to the whole tooth a triangular outline, as in 

 Lagopsis. In Prolagns aniingensis (PI. 37. fig. 9) this cusp is situated near or close 

 to the middle line ; in P. sanlns *, of which I have examined hundreds of specimens, 

 its position is nearer the inner side. As before mentioned, in very rare cases of 

 P. sardiis, this usually isolated cusjj is united with the tooth near the inner side, as in 

 d. 2 of fig. 5. In other cases of P. oenuigenals (fig. 12, PI. 37-) and P. sardus, it may 

 be united with the tooth near its outer side. This latter fusion I found to have 

 taken j^lace in 10 specimens of ytTH out of 575 examined, from the ossiferous breccia 

 of Monte San Giovanui (Sardinia) (P. sardus), and in two cases out of 84 examined 

 from Toga, near Bastia (P. sardus, var. corsicanus). The cusp was united with the 

 tooth near the iuuer side in two of tlie 575 examples from Monte San Giovanni. 

 Cusp " ^" I have met with only in p. 2 of Frolagus ehaiiiis (page 460). 



A comparison witli the specimens before described shows the usually odd isolated cusp 

 to be the homologue of the " paracouid " combined with the anterior odd cuspidule of 

 Lagopsis, while the outer cusp of the tripartite anterior lobe is jiresent, also, in p. 2 ; in 

 P. ceniiigrnsis it is generally stouter than tlie outer cusp (6) of the median lobe, whereas 

 in P. sardus the inverse is the rule. In exceptional cases of P. sardus 1 find this outer 

 cusp of the anterior lobe completely isolated, as it is in the deciduous tooth of fig. 5. 



A second characteristic feature of the pTs of Prolagus (figs. 9, 12) is a longitudinal 

 enamel-fold, filled with cement, which, beginning from behind the isolated anterior 

 cusp, proceeds backward to near the hinder margin of the tooth, thus completely dividino- 

 the middle lobe into an outer and an inner cusp, and incompletely so the posterior one, 

 on which it also encroaches. The longitudinal arrangement of the elements of this jj. 2 

 of Prolagus. in opposition to the transverse arrangement of the posterior teeth, is very 

 striking. 



I now proceed to a consideration of the same tooth in the Leporidse. With reference to 

 p. 2 of Palceolagus, Leidy states: — "The anterior four inferior molars [of Paheolayus^ bejr 

 a near resemblance in Ibrm and constitution Avith the corresponding series of TUauonigs 

 visenoviensis, as represented in j)!. 46 of Gervais' Zool. et Pal. Fv." f. Comparing it with 

 the same tooth in Lepus, Leidy further says in the original description of Paheo- 

 lagus : — " The first inferior molar is bilobed, and not trilobed as in the latter (Lepus) " j. 

 In his second memoir the first inferior molar of Pakeolagus is said to be composed of a 

 double column as in the others, the same tooth in the Hare of a triple column §. Cope 



* R. Hensel, l. c. pi. xvi. fig. S. 



t ' Extinct Mammalian Fauna of Dacota and Xebraska,' p. 333, pi. xxvi. (iSii'j). 



t Proc. Ac. Philad. p. 89 (18.56). 



§ Extinct ilamm. Fauna, &c., p. 331. 



