EVOLUTION OP THE SKULL AND TEETH OF EOCENE TITANOTHERES 



307 



near the same place. An occiput in the Princeton 

 Museum (No. 10044), also from Cottonwood Creek, 

 which was described and figured by Earle (1892.1, 

 p. 353) as L. laticeps, may also be provisionally referred 

 to L. priscus. 



The generic or LimnoJiyops character is shown in the 

 presence of a hypocone on m^. Divergence from the 

 type of L. laevidens is indicated by the elongate form 

 of p^ as distinguished from the rounded or transversely 

 oval form which this tooth presents in L. laevidens. 

 The progressive stage is indicated by the rudimentary 

 condition of the tritocone on p' in the type specimen 

 of L. priscus. 



The type sTcull. — The skull of the type specimen 

 (fig. 259) is somewhat larger than that of the American 

 tapir. It is extremely interesting to note that it more 

 closely resembles that of the common ancestral form of 

 the titanotheres than does the skull of the contem- 

 porary species Palaeosyops paludosus. The 

 skull and m^ are therefore more primitive 

 than those of Palaeosyops. The specimen in 

 hand is much crushed laterally, and the 

 restoration represented in Figure 259 is an 

 approximation to the complete form. The 

 estimated total length from condyles to 

 symphysis is 375 millimeters; from the crest 

 of the occiput to the tip of the nasals 395. 

 The cranium shows the typical LimnoJiyops 

 characters enumerated above — namely, 

 short supraoccipital exposure on top of the 

 skull; moderately high, thin sagittal crest, 

 which diverges into the supratemporal crests 

 about 95 millimeters in front of the occiput; 

 occiput moderately high (proportions not 

 to be exactly ascertained owing to its crushed 

 condition). The nasals measure 143 milli- 

 meters (ap.) and taper gradually toward the 

 extremities. In the base of the skull the 

 paroccipital processes are separated by shal- 

 low grooves from the post-tympanic processes, and the 

 external auditory meatus is apparently open below. 



Dentition. — The superior teeth are well represented 

 in the type cranium. The lateral incisor is enlarged 

 as in Palaeosyops; the canine is slender, slightly 

 recurved, and followed by a very narrow diastema; 

 the grinding series (Pis. LVI, LX, LXII) is con- 

 tinuous, p'-m^ measuring 149 millimeters; pm^ pre- 

 sents a very large convex protocone and rudimentary 

 tritocone. The inferior teeth are well shown in the 

 cotype jaw (Am. Mus. 11688), found not far from the 

 skull and probably belonging to the same individual. 

 The two incisors preserved are cingulate posteriorly; 

 the canine is rather slender, laterally compressed, and 

 slightly recurved; behind this is a small, simple pi, 

 followed by a narrow diastema; p2 and ps are com- 

 paratively narrow and simple teeth, exhibiting ex- 

 tremely rudimentary paraconids and slightly cres- 



centic hypoconids, and a faint rudiment of the meta- 

 conid in ps; p4, on the contrary, is submolariform, 

 exhibiting a well-developed metalophid and a rudi- 

 mentary hypolophid or posterior crescent which lacks 

 only the entoconid. The three true grinders are 

 simple, with rudiments of vertical striations, with 

 distinct paraconids, and with a sharply pointed sub- 

 crescentic hypoconulid on la^. The characters of the 

 jaw are well shown in Figure 259. 



Back of the cranium. — The back of a cranium in the 

 Princeton collection (Princeton Mus. 10044) may be 

 provisionally referred to this species. It was found 

 on Cottonwood Creek, Bridger Basin, Wyo., in the 

 Bridger formation, level B 3(?) by Francis Speir, of the 

 Princeton expedition of 1877, and was rightly referred 

 to Limnohyops by Earle. The deep (45 mm.) and nar- 

 row (9 mm.) sagittal crest is not quite so elongate as 

 that of the Yale Museum type of L. laticeps, the supra- 



FiGUKB 259. — Skull of Limnohyops priscus 



One-fourth natural size. Skull Am. Mus. 11687 (type); Grizzly Buttes (west), Bridger Basin, Wyo.; 

 Bridger formation, level B 2. Partial reconstruction of crushed skull made by L. M. Sterling 

 under the direction of W. K. Gregory. Lower jaw. Am. Mus. 11688, possibly belonging to same 

 individual. Details of zygoma restored from Am. Mus. 5104 (type of L. laevidens); details of 

 occiput from Princeton Mus. 10044, L. priscus. 



temporal ridges beginning to diverge rapidly 77 milli- 

 meters in front of the crest of the occiput. In 

 addition to the points noted in the type skull above 

 described, we observe in this occiput (1) two large 

 mastoid foramina, (2) two prominences just above the 

 foramen magnum, (3) the broadly extended (48 mm.) 

 post-tympanic processes closely conjoined at the base 

 with the relatively narrow (12 mm.) paroccipital proc- 

 esses; (4) the relatively broad (37 mm.) postglenoid 

 processes. In palatal view the zygomata are seen to 

 have a transverse measurement of 268 millimeters, the 

 post-tympanic and postglenoid processes are slightly (3 

 mm.) separated; the articular facets for the condyles 

 of the jaw are nearly transverse; a bridge of bone, 19 

 milluneters in width, separates the foramen ovale and 

 foramen lacerum medium. These features are well 

 illustrated in Figure 260 as compared with similar views 

 of L. laticeps and L. matthewi. 



