EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 



399 



moderately long, recurved, sharp edged, and may have 

 been used in fighting, as by the existing camels; the 

 offensive power of the front teeth was less, however, 

 than in the short-jawed Palaeosyops. The cheek 

 teeth, concerned in the comminution of food, were 

 relatively long crowned, with pointed cusps, and 

 constituted an elaborate cutting and triturating appa- 

 ratus. The movement of the more slender mandible 

 was partly vertical, partly oblique, since the wearing 

 of the cheek teeth gives evidence of an oblique shear- 

 ing action. Adapted to these conditions were the 

 length and proportions of the chewing muscles and 

 their angles of action. (See below for details.) 



It is therefore probable that since the food evidently 

 required finer cutting and better trituration than 

 the food of Palaeosyops, Dolichorhinus was either 

 a browser on harder materials or a grazer, perhaps 

 coming out from the forests at night into the open 

 grassy places or searching for smaller twigs, like 

 the Indian rhinoceros {R. unicornis). The bend- 

 ing down of the facial upon the cranial axis is a 

 characteristic of many grazers, whereas the bend- 

 ing up of the facial axis is generally characteristic 

 of browsers. 



Directing attention, on the other hand, "to the 

 progressive backward shifting of the hinder border 

 of the posterior nares to what is known as the "sec- 

 ondary palate," Riggs (1912.1, p. 36) has advanced 

 the hypothesis that DolichorMnus was a river-fre- 

 quenting form which perhaps fed upon submerged 

 plants, like the moose. The backward shifting and 

 closure of the hinder border of the palate is an ob- 

 vious advantage to animals feeding partly in the 

 water and is characteristic of many water-living 

 forms. 



General characters of the genotype, D. hyognathus. — 

 The elongate skull, the broad, flattened, and suture- 

 less cranial region, the elongate nasofrontal horns 

 are characters partly of progressive dolichocepha- 

 ly, partly of family affinity to the Oligocene forms. 

 The features of the main line of Dolichorhinus are 

 the extreme narrowing and lengthening of the 

 skull and zygomatic arches, the convex upward 

 arching instead of a concave saddle shape of the 

 skull top, the broad infraorbital shelf, the shal- 

 low jaws, the parallel series of grinding teeth, and 

 especially the extremely long, narrow nasals. The 

 horns are borne chiefly on the nasals, as in Mesatirhi- 

 nus, in contrast with Manteoceras, in which they are 

 borne chiefly on the frontals. The occiput is low, 

 possibly in correlation with the bending down of the 

 cranium. In palatal view we observe the marked 

 backward extension of the posterior nares and the 

 formation of a secondary palate. The jaw is dis- 

 tinguished by its long, slender, recurved coronoid 

 process and its depressed angle. These characters 

 combine to constitute this animal one of the most 

 peculiar and distinctive of the whole titanothere 



At a first glance the long skull suggests that of a 

 horse, but a closer examination shows that, although 

 both are dolichocephalic, the resemblance is entirely 

 superficial; the horse has a primitive short cranium 

 (brachycrany) and an enormously long face (dolichopy) 

 or preorbital region. Dolichorhinus has an elongate, 

 highly modified cranium (dolichocrany) and postor- 

 bital region and a relatively short face (brachyopy). 

 As compared in detail with the skull of a horse that of 

 Dolichorhinus furnishes an instructive minsrlins: of 



craruzan. 



FiGUBE 337. — Skulls of Dolichorhinus hyognathus (A) and modern 

 horse (B) 



One-sixth natural size. Tliese show analogous and divergent adaptations to grazing 

 habits. A-A', Basicranial axis; B~B^, basipalatal axis. 



convergent resemblances to other long-headed un- 

 gulates and divergent hereditary differences. Among 

 the convergent resemblances in Dolichorhinus are 

 (1) the lengthening of the whole skull, especially of the 

 face; (2) the bending down of the anterior half of the 

 skull; (3) the backward prolongation of the palate; 

 (4) the semicircular or cropping arrangement of the 

 incisors; (5) the prominence of the orbits; (6) the 

 forward extension of the masseter muscle, the anterior 

 slip in Dolichorhinus being attached to the infra- 

 orbital shelf. 



Among the divergent hereditary differences charac- 

 teristic of the titauotheres and shown in Dolichorhinus 



