SKCiiM) OUTLINK OF TUITriJKKCl'I.V 85 



which this molar belonged as no two series are modified similarly in 

 all these respects. Yet the prevailing method among many pal;i-<ui- 

 tologists is to pass lightly over most of the differentia and, for example, 

 group widely divergent forms under the Lophiodontidse as if in the con- 

 stitution of these dense enamelled tissues nature could lightly pass from 

 one to another. 



A few words now upon the secondary "styles."' Their function is 

 evidently to increase and elaborate the crushing surface of the crown. In 

 Pht ii'irot/i/x the first to appear is the " mesostyle " between the paracone 

 and metacone, but this genus was on a side line of the Condylarthra. In 

 all true perissodactyls and artiodactyls, the first peripheral cusp to appear 

 is the antero-external buttress of the upper molars, which we call the 

 " parastyle," since it adjoins the paracone. The " mesostyle " appears 

 later, and only in those ungulates in which the paracone and metacone 

 are moulded into crescents. Thus the lower Eocene Hyracotherium does 

 not exhibit this cusp, but it appears as a distinctive feature of the middle 

 and upper Eocene Pachynolophus (Orohippus). The mesostyle was strongly 

 developed in all the selenodont, buno-selenodont and lopho-selenodont 

 types, such as the Artiodactyla and J/o//.srn/A ( r/>?tt,, Chalicotherium, 

 Palceosyops, the palseotheres and horses. Look at an upper molar of 

 Merych-ippus and see what an important role these styles play (Fig. 162). 

 First, we observe the " parastyle " and " mesostyle," next most important 

 is the " hypostyle," which develops near the hypocone upon the posterior 

 cingulum of Mesohippus and Anchitherium and finally completes the 

 border of the ''anterior fossette " or cement lake. The horse molar, by 

 the way, furnishes the best illustration of the value of tracing back the 

 various portions of the crown to their birth-place in the primitive crown 

 of Hurocothi'iitim. Every turn in this labyrinth of folds is thus made 

 perfectly clear. 1 



A corresponding set of styles grows ap on the lower molars, and it is 

 very easy to locate them with reference to the reciprocal upper set if we 

 simply keep in mind the fact that throughout the whole course of develop- 

 ment the elements of each trigonid are placed just in front of those of the 

 corresponding trigon, that is, the protoconid and uietaconid fit iust in front 

 of the paracone and protocone, as shown in the diagram (Figs. 37, 396-). 

 Thus the inferior entostylid is developed near the entoconid, while the 

 superior hypostyle develops near the hypocone. The first of the inferior 



'"' [The para* and metastyles as well as the proto- and metaconules are very ancient 

 elements of the molar crowns, since they appear in the Upper Jurassic and Upper 

 Cretaceous trituberculates. The termination "style" as used in this book is applied (1) 

 to all cusps originally external to the para- and metacones ; (2) to the " protostyle," 

 "hypostyle," "entostylid," etc. ED.] 



1 Mr. Lydekker has courteously called attention to the fact that in the earlier study 

 of this subject the writer misinterpreted the descriptive terms employed by Huxley. 



