310 



TITAKOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



occiput very high; cranium relatively elongated, 

 with space (4 mm.) between post-tympanic and post- 

 glenoid processes; temporal openings as defined by 

 zygomatic arches elongated. 



This type (Am. Mus. 11679) is an exception to all 

 the other species referred to Limnohyops in the ab- 

 sence of the hypocone on m^. The specific name, 

 monoconus, refers to the existence of but a single 

 internal cusp (protocone) on this tooth. The animal 

 is provisionally referred to the genus Limnohyops on 

 strong evidence in five other points of cranial structure 

 which are cited below. 



As compared with the type of L. matthewi, the 

 animal on which this species is founded is of very 



FiGUBE 263. — Skull of Limnohyops monoconus 



Palatal view. One-fourth natural size. Am. Mus. 11679 (type); Grizzly Buttes, 



west Bridger Basin, Wyo.; Bridger formation, level B 2. 



robust size; its size is especially robust for the low 

 geologic level from which it is recorded — namely, B 2. 

 Its proportions agree fairly well with those of L. lati- 

 ceps, which is referred to level D 2. 



Materials. — The species is certainly Icnown only 

 from the type (Am. Mus. 11679), which consists of 

 the crushed cranium of a robust male specimen, as 

 indicated by the large, recurved canines. Other 

 doubtfully referred specimens (Am. Mus.' 5102, 12680) 

 are recorded from B 5. 



As the hypocone on m^ is the alleged "generic" 

 character of LimnoTiyops and is present in the other 

 species L. laevidens, L. matthewi, and L. laticeps, its 



absence in this form is very exceptional and causes 

 some doubt as to the propriety of the generic refer- 

 ence of this species to Limnohyops. The other grounds 

 for referring this animal to Limnohyops rather than to 

 Palaeosyops are strong, however — namely, (1) the 

 doubly ridged ectolophs of p"-p*; (2) the very elevated 

 occiput; (3) the more or less angulate form of the 

 suborbital bridge in the malars; (4) the splint of the 

 maxillary extending on the lower side of the malar 

 bridge; (5) nasals not perceptibly narrowing anteriorly. 



Specifically this animal is readily distinguished from 

 all other species of Limnohyops by the absence of the 

 hypocone on m^. As compared with L. laevidens it is 

 an exceptionally large form, the measurements (esti- 

 mated) being, from the condyle to the incisive border 

 439 millimeters, width across the zygomata 320. It 

 is also distinguished by the very robust and recurved 

 canines. From L. laticeps it is distinguished by the 

 higher occiput, by the absence of a hypocone on m^, 

 and by its supposed lower geologic level. It is dis- 

 tinguished from L. matthewi by the more elongate 

 cranium, correlated with which are the oval openings 

 circumscribed by the zygomatic arches, and by the 

 separation between the postglenoid and post-tympanic 

 processes. 



The skull is of massive proportions, with widely 

 arched zygomata (320 mm., estimated) as compared 

 with the total length (439 mm., estimated). (See fig. 

 263.) The anterior portion is too much crushed for 

 recognition, except that the nasals have the form char- 

 acteristic of Limnohyops, with more parallel sides than 

 in Palaeosyops. Seen from behind the occiput is more 

 elevated than in L. laticeps and resembles that of 

 L. matthewi. (See fig. 262.) The extreme height of 

 the sagittal crest above the foramen magnum is 133 

 millimeters, and above the bottom of the condyles 

 179; the condyles measure 99 millimeters in width. 

 Seen from above the nasals are narrow and long, 

 measuring 175 to 186 millimeters as compared with 

 168 in L. laticeps. In the region of the frontonasal 

 horn swelling there is a rugose area which may have 

 exhibited a rudimentary frontonasal horn. 



Dentition. — The canine is exceptional in its robust 

 size and recurved form, the height being estimated at 

 41 millimeters and the diameters at the base of the 

 enamel being, anteroposterior, 25; transverse, 25. 

 The crown approaches that of Manteoceras in the 

 swelling of the base. The premolars are primitive 

 in lack of complication: (1) there is a small trito- 

 cone on p^, (2) p^ and p^ lack the internal cingula 

 entirely, a primitive condition, (3) the double ridging 

 of the ectolophs is a Limnohyops character. The mo- 

 lars, m'-m^, measuring 93 millimeters, are very distinc- 

 tive in form, with oblique ectolophs and prominent 

 parastyles. The triangidar m^ especially is of quite 

 different form from that of L. matthewi, the tooth 

 narrowing toward the inner side and having a long 

 oblique outer border. 



