6 



UNITED STATES NATIONAL MUSEUM BULLETIN 294 



concave and ovoidal Sylvian apophysis, which articu- 

 lates with tlie licad of tiie stapes. Absorption of the crus 

 longuni has reduced the body of the incus. The crus 

 breve is short, conical and acuminate, and is projected 

 at a right angle to the crus longum. A small facet on the 

 dorsal surface of the apical portion of the crus breve 

 serves as the area of contact with the fossa incudis of the 

 periotic. 



From the apex of the crus longum to the base of the 

 body, the incus measures 7.4 mm., and the greatest di- 

 ameter of the base is 6 mm. 



Stapes.— The stapes (USNM 23794; pi. 2, fig. 3) has 

 the normal balaenopterine form; it is narrower at the 

 base and slightly more elongated than that of Meto- 

 /lorrfu-s duriniums (Kellogg, 1968, pi. 48, fig. 2). The 

 circular intercrural aperture connecting elongated cav- 

 ities on opposite sides is open and not partially obstruct- 

 ed. The crura are straight and less divergent than on 

 the stapes of Boda-enoptera acutorostrata (Doran, 1878, 

 pi. 02, fig. 31). An oval concavity or umbo occupies the 

 vestibular face of the footplate of the stapes. The foot- 

 plate liad been displaced from the fenestra ovalis prior 

 to removal of the enveloping matrix. A well-defined 

 scar on the posterointernal angle below the head marks 

 the area of attachment of the stapedial muscle. An 

 ovoidal facet on the head of the stapes marks the area 

 of contact with the corresponding facet on the head of 

 the crus longum of the incus. 



The greatest length of the stapes is 7.8 mm., and the 

 greatest diameter of the footplate is 4 mm. 



MANDIBLE 



The left mandible of the type-specimen (USNM 23794) 

 lacks a section approximately 290 mm. long near the 

 anterior end. Slender branches of the roots of a tree 

 growing in the earth immediat/ely above this skeleton 

 had penetnited into tlie mandibular canal through the 

 external mental foramina and in the course of their en- 

 largement during growth disrupted and disintegrated 

 a lengthwise strip of bone in which the orifices of these 

 foramina were located. Hence the number and position 

 of these foramina cannot now be determined. 



There is a noticeable flattening of the anterior two- 

 thirds of the internal surface of this mandible ( fig. 1 ) , 

 in contrast to the dorsoventral convex curvature of the 

 external surface. Ventrally the external surface of the 

 horizontal ramus meets the internal surface to form a 

 well-defined angular edge, but not comparable to the 

 elevated dorsal ridge or rim which is quite thin pos- 

 teriorly and extends forward about 400 mm. in front 

 of the coronoid process and also progressively increases 



in thickness. Below and internal to the base of this dorsal 

 ridge, about 100 mm. anterior to the apex of the coro- 

 noid process, the orifice of the posteriormost small in- 

 ternal nutritive foramen is located. This lengthwise 

 series of small foramina, separated by intervals that 

 progressively lengthen from 15 to 60 mm., rise to the 

 dorsal edge of the mandibular ramus anteriorly, the 

 terminal foramen opening into the long (140 mm.) an- 

 teriorly directed narrow and rather deep groove. 



On the left mandible (USNM 22961) tentatively re- 

 ferred to this species, the posteriormost internal nutri- 

 tive foramen is located 1100 mm. behind the anterior 

 end of the mandibular ramus; the largest of these small 

 internal foramina is located 9 mm. below the edge of 

 the thin dorsal rim. There are nine mental foramina 

 along the outside curvature of the anterior 860 mm. of 

 this left mandible, each of which opens into an ante- 

 riorly directed groove of variable length. These mental 

 foramina are located behind the anterior end of the 

 mandibular ramus as follows: flirst 275 mm.; second 

 100 mm. behind the first ; third 60 mm. behind the sec- 

 ond; fourth 45 mm. behind the third; fifth 50 mm. 

 behind the fourth; sixth 70 mm. behind the fifth; 

 seventh 95 mm. behind the sixth ; eighth 60 mm. behind 

 the seventh; and ninth 90 mm. behind the eighth. A 

 large terminal mental foramen is present below the long, 

 narrow dorsal groove on this referred mandible, but is 

 closed on the type mandible. 



Above the ventral edge of the anterior end of the 

 mandibular ramus and below the short longitudinal 

 crease, the flattened lower border (measuring 35 mm. 

 dorsoventrally on the type mandible) of the internal 

 surface is depressed. 



Although the apex of the .small coronoid process is 

 eroded, the curvature of the preserved portion shows 

 that it was low, subtriangular and everted apically, con- 

 cave internally and convex externally, and also located 



Figure 1. — Cross-sections of left mandible, USNM 23794, of Thinocrtus 

 arthritui. a, 100 mm. behind anterior end; b, 1100 mm. anterior to 

 hinder articular surface of condyle; c, 700 mm. anterior to hinder 

 articular surface of condyle; d, 360 mm. anterior to hinder articular 

 surface of condyle. 



