D)'. C. I. Forsyth Major — Pliohyrax from Samos. 549 



So far back as 1873 I described the outer parts of upper molars as 

 being composed of six cusps, which in my figures were designated 

 by the letters a, b, c, d, e, f, proceeding from before backward, and 

 I added that all the differences were to be traced back to the 

 respective position of these six parts to each other and to their 

 relative development.^ When criticizing these views, Schlosser 

 maintained that a, c, d, and / are not essential parts of the tooth, 

 but mere secondarily developed crests or tubercles.'- However, as 

 Winge had shown in the meantime, in such low mammals as 

 Insectivora and Polyprotodont Marsupialia, these cusps are arranged 

 in two longitudinal series, much more independent of each other 

 than in higher Mammalia. He admits five cusps, and numbers them 

 from before backward 1, 2, 3, 4, 5 ; cusp 2 corresponding to my 

 <;, d. The first three (1, 2, 3) form the outer, the latter two (4, 5) 

 the inner series, and he further states that the former are the 

 original three cusps of the Mammalian molar, the homologues of 

 the three cusps of Triconodonts.^ At any rate they are remarkably 

 -developed in several upper Cretaceous Mammalia from the Laramie.* 

 According to the nomenclature of Osborn, who in his turn considers 

 the outer cusps (-styles) as secondary formations, the five cusps are, 

 always proceeding from before backward, the parastyle, paracone, 

 mesostyle, metacone, metastyle, or, considering their arrangement in. 

 two longitudinal series : parastyle, mesostyle, metastyle (outer series) ; 

 paracone, metacone (inner series). 



In Ungulates these diiferent parts are connected together, so that 

 the tooth presents a W-shaped outer wall, usually shallow. 



F. M. (1873). Wiuge (1882). Osboru. 



^ t'<^ J- ■> ^. s. jjarasf^/Je. mesc,':fy^e- TTze&isfi^le. 



The more a tooth is hypselodont, the more its outer wall becomes 

 flattened, and the folds dividing the cusps tend to disappear; as 

 may be seen by comparing my figures of brachyodont (Figs. 4, 5) 

 with those of hypselodont (Fig. 3) Hyraces, in which I have adopted 

 Winge's mode of numbering the cusps. In the brachyodont teeth 



1 and 2 are very prominent, and in both brachyodont and 

 hypselodont Hyraces the fold dividing 2 and 5 is particularly shallow, 

 and tends to disappear in the course of wear, at an earlier date in 

 iiypselodont, very late in brachyodont Hyraces ; so that very often 



2 and 5 become fused together, as is the case in m. 1 of Fig. 3 (in 

 the figure this fusion is not so apparent as in the original). The 

 ifinal result is that in worn molars the fold between 4 and 2 seems 

 to divide the outer wall of the tooth in two equal halves ; but the 



1 Palaeontographica, vol. xxii (1873), p. 101, pi. vi. 

 - Morphol. Jahrb., xii (1887), p. 102. 



3 Vidensk. Meddel. Natiu-h. Foren. Kjobenhavn, 1882, p. 15, pi. iii. 

 * H. F. Osborn, " Fossil Mammals of the Upper Cretaceous Beds" : Amer. Mus. 

 !Nat. Hist, and Art, vol. xvii (1893), pi. viii. 



