308 



TIT.\KOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



Limnohyops matthewi Osborn 



Text figures 116, 261, 262 



[For original description and type references see p. 180] 



Type locality and geologic horizon. — -Grizzly Buttes 

 (west), Bridger Basin, Wyo.; Bridger formation, 

 Palaeosyops palvdosus-OroMppus zone, level B 2. 



Specific characters. — Type of intermediate size as 

 compared with L. laevidens and L. monoconus. M} 

 small, with large hypocone and quadrate inner half. 

 Occiput very high and narrow. Cranial portion of skull 

 greatly abbreviated, bringing post-tympanic and post- 

 glenoid processes into broad union. Temporal open- 

 ings subcircular as defined by zygomatic arches. (See 

 figs. 261, 262.) 



Figure 260. — Back part of skull of Limnohyops priscus 



One-fourth natural size. Princeton Mus. 10044; Cottonwood Creels:, Bridger Basin, Wyo.; lower part of Bridger formation. 



Ai, Side view; Az, basal view; A3, top view; A4, occipital view. 



This species is named in honor of Dr. W. D. 

 Matthew, of the American Museum, who has done 

 so much to advance our knowledge of the Bridger 

 fauna and geology. 



As compared with the primitive type skulls of L. 

 laevidens and L. priscus, above described, the cranial 

 region of this animal is surprisingly specialized in its 

 abbreviation, or extreme brachycephaly. This pro- 

 gressive character is difficult to reconcile with the fact 

 that it is geologically recorded in the same low level as 

 that of L. laevidens. 



Materials. — The type (Am. Mus. 11684), consisting 

 of the posterior portion of a skull which includes the 

 orbits, is the only specimen at present referable to this 

 species. It was found in 1903 by the American 

 Museum expedition. 



Distinctive characters. — The brachycranial type of 

 this species is readily distinguished from that of 

 L. laevidens by the very specialized condition of the 

 posterior portion of the skull, including the elevated 

 occiput and the firm inclosure of the auditory meatus 

 below. It differs from L. laticeps when seen from 

 above in the greater height of the occiput and in the 

 transversely oval form of the temporal openings as 

 defined by the zygomatic arches. It differs from 

 L. monoconus in the presence of a large hypocone on 

 m' and in the quadrate form of the inner side of this 

 tooth, also in the rounded temporal openings. 



In the lateral view of the skull we observe that the 

 zygoma descends rapidly anteriorly and thins out as 

 it passes into the anterior 

 portion of the malar, which 

 constitutes the suborbital 

 bridge; this bridge is de- 

 pressed in section but ex- 

 hibits a rounded rather 

 than angulate outer bor- 

 der. This species differs 

 from Palaeosyops in that 

 the masseteric insertion 

 ridge is not carried for- 

 ward any great distance, 

 and that there is a slender 

 splint of the maxillary ex- 

 tending back below the 

 malar bridge. The sagit- 

 tal crest extends 50 milli- 

 meters above the brain 

 case proper; it is even 

 higher and thinner than in 

 L. laticeps. As seen from 

 below the temporal open- 

 ings are transversely oval, 

 the anteroposterior meas- 

 urement being 82 milli- 

 meters, and the transverse 

 measurement from the 

 malars to the alisphenoids 

 95. The sagittal crest divides these great insertion 

 cavities of the temporal muscles and is thin at the 

 summit, measuring 8 millimeters at the thinnest point; 

 it is also elongate, extending 94 millimeters from the 

 tip of the occiput to the point where it bifurcates into 

 the supratemporal ridges. 



The chief feature of the occiput is the foreshortening 

 and compression of its lower portion against the back 

 portion of the squamosals and zygomatic arch, 

 causing a broad junction of the postglenoid and 

 post-tympanic processes and a very characteristic 

 flattening of this region. Seen from behind (fig. 262) 

 the occiput measures 116 millimeters above the 

 foramen magnum, and 157 above the bottom of the 

 occipital condyles. Conforming to the smaller size 

 of the skull as a whole, the condyles measure 86 milli- 



