Auditory Region of Typotherid 



87 



in place in the vagina of the left side. Van Kampen states that 

 the stylomastoid foramen in T. cristatum lies between the tympanic 

 and the post-tympanic process. This is correct but his figure would 

 indicate that what he identified as the foramen is really the vagina 

 processus hyoidei. 



The epitympanic sinus is very large and hollow; it is noticeable 

 externally on the occipital surface. Roth (1903, Plate 3, Fig. 4) 

 gives an excellent cross-section figure of the sinus in T. cristatum. 



The mastoid process is visible on the occiput as a thin strip of 

 bone dorsal to the exoccipital. It merges laterally with the post- 

 tympanic process of the squamosal. This latter feature suggests 

 the possibility that the "adventitious bone" previously described 

 by me (1932, p. 19) may represent an enlarged, plate-like mastoid 

 process. This does not seem probable, however, in view of the fact 

 that the mastoid process in the Toxodonta and Typotheria is known 

 to be of small size. Furthermore, if this supposed neomorph (which 

 only occasionally appears) was in reality the mastoid process one 

 would expect to find the suture between it and the post-tympanic 



Fig. 9. Pseudotypotherium pseudopackygnathum (Ameghino). Occipital region of left side. 

 A.M. No. 14509. x 2/3. cT.m., crista meati; ept.sin., epitympanic sinus; mas., mastoid; p.a.e., poms 

 acusticus externus; p.occ.pr., paroccipital process; p.ty.pr., post-tympanic process. 



process in the majority of young skulls. Actually the reverse is 

 the case (see, in this connection, Van Kampen's figure of the occiput 

 of a young Toxodon platensis, 1905, p. 618, fig. 79). 



Elements touching the external surface of the bulla. — Anteriorly 

 the bulla sends forward a spine (see above) which extends between 

 the ali- and basisphenoid ; laterally it is in contact with the alisphe- 



