DISTINCTIVE CHARACTERS OF MASTODON AND ELEPHANT. 19 



its reception among them. Two of the Loxodons, namely, E. 

 (Lox.) planifrons and E. (Lox.) Africanus, have a ridge- 

 formula which is identical or nearly so with that of Stegodon 

 insignis ; but the separation of the group is indicated by the 

 great increase of vertical height in the colliculi, and by the 

 layers of enamel assuming the character of plates, instead of 

 the mastoid eminences of Stegodon. E. {Lox.) meridionalis 

 has a higher number of plates in the ' intermediate molars ' 

 than those two species, and constitutes a frontier form, lead- 

 ing towards the next group, Euelephas. But the ridge- 

 formula in this form would appear to be hypisomerous, and 

 the aggregate characters indicate its position among the 

 Loxodons. The majority of the species in the group Euele- 

 phas are well marked by the progressive increments and high 

 numerical expression of the crown -ridges of the intermediate 

 molars, by the great vertical height of the colliculi, and the 

 attenuated plates of enamel. One species among them, E. 

 (Eueleph.) Hysudricus, constitutes a frontier form leading 

 towards E. (Lox.) meridionalis. More ample details respect- 

 ing the Elephants will be given in the second part of this 

 memoir, when treating of the European fossil species. 



To revert specially to the Mastodons, Trilophodon and Te- 

 tralophodon (including under the latter the exceptional five- 

 ridged Mastodon Sivalensis), as regards the number of forms 

 at present known, are of nearly equal value, the former in 

 our view comprising 7, and the latter 6, well-marked species ; 

 and they are each divisible into two parallel subordinate 

 groups, the exact appreciation of the characters of which is 

 of much service in the determination of the European fossil 

 species. In the one series, the ridges are broad, transverse, 

 more or less compressed into an edge, with the valleys open 

 throughout and uninterrupted by subordinate tubercles : 

 these are well represented in Trilophodon by M. (Triloph.) 

 Ohioticus, and in Tetralophodonbj M. (Tetraloph.) latidens. In 

 the other series, the ridges are composed of blunt conical 

 points, which are fewer in number, more elevated, and flanked 

 in front and behind by one or more subordinate outlying 

 tubercles, which disturb the transverse direction of the 

 ridges and block up the valleys, interrupting their continuity 

 across. This series is represented in Trilophodon by the 

 Miocene European species, M. (Triloph.) angustidens, and in 

 Tetralophodonhjth.e Pliocene species,lf. (Tetraloph.) Arvernen- 

 sis of the Crag. (See Plates III. & IT.) The species with 

 transverse compressed ridges, in both subgenera, may be 

 compared with Dinotherium as regards their molar crowns, 

 and the other series with Hippopotamus. 



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