84 Field Museum of Natural History — Geology, Vol. VI 



which will shortly be published, show that a septum bullae is present 

 in the genera investigated. The bullae of the specimen under 

 discussion have not been opened, but, to judge from the conditions 

 observed in the closely related Pliocene genus Typotheriopsis, are 

 probably compound and composed of an entotympanic in addition 

 to the tympanic. In Typotheriopsis the sinus hypotympanicus is 

 finely cancellous while the cavum tympani is hollow; in the American 

 Museum specimen of P. pseudopachygnathum cancellae are visible 

 in the broken anterior portion of the right bulla. The ventral wall 

 of the bulla in T. cristatum, as figured by Roth (1903, Plate 3, Fig. 4), 

 is very dense and thick. I have observed the same condition in 

 Typotheriopsis. 



Van Kampen's description of the external appearance of the 

 bulla of T. cristatum applies almost equally well to the specimen 

 under consideration. Some minor differences may, however, be 



fov.t-fl.rrt. 



p.occ. pr. 



pty.pr. 



Fig. 7. Pseudotypotherium pseudopachygnalhum (Ameghino). Basicranial region of left side, 

 ventral view. A.M. No. 14509. x 2/3. c.f., condylar foramen; car./., carotid foramen; cr.m., crista 

 meati; f.l.p., foramen lacerum posterius; f.ov. + f.l.m., foramen ovale and foramen lacerum medium; 

 p.gl.f., post-glenoid foramen; p.gl.pr., post-glenoid process; p.occ. pr., paroccipital process; p.ty.pr., 

 post-tympanic process; iykl., tympanohyal; v.pr.hy., vagina processiLs hyoidei. 



noted. The ventral margin of the bulla bears a slight keel, more 

 prominent on the right than on the left, which merges anteriorly 

 with the styliform process. There appears to be no distinct process 

 from the ventral margin abutting against the paroccipital process 

 in this species, the junction between tympanic and paroccipital 

 process being practically a straight, transverse line. The two bones 

 are in simple contact and do not present an interlocking sutural 

 union as they do in certain interatherids. The spine, by which the 



