

Auditory Region of Typotherid 85 



bulla comes in contact with the basisphenoid, is somewhat longer 

 and narrower in P. pseud opachygnathum than in T. cristatum. It 

 extends anteriorly as a thin wedge between the basisphenoid and the 

 alisphenoid. 



There is no definite line of demarcation on the ventral surface 

 of the bulla which would indicate the limits of the entotympanic 

 and the tympanic. From a comparison with Typotheriopsis, however, 

 it may be stated with some confidence that the anterior and mesial 

 thirds of the bulla are formed by the entotympanic. 



The tubular meatus is large, and runs postero-externally in a 

 slightly upward direction. The porus acusticus externus is circular 

 and bounded dorsally and posteriorly by the squamosal, anteriorly 

 and ventrally by the tympanic. The ventral border is formed by a 

 lip projecting posteriorly from the crista meati,^ a feature charac- 

 teristic of the Toxodonta and Entelonychia but lacking in the 

 Interatheriidae and Hegetotheriidae, due to the absence of a well- 

 defined crista meati in members of these families. The portion 

 of the tympanic forming the anterior boundary of the porus is 

 firmly fused with the post-glenoid process, as was noted by Van 

 Kampen in T. cristatum. This author goes on to state that there 

 are two ridges on the under wall of the meatus in this form, of which 

 the anterior passes into the glenoid fossa. The posterior ridge, 

 according to his description, lies behind the stylomastoid foramen 

 (see below) and lateral to the paroccipital process; it extends upwards 

 behind the porus, forming the external portion of the occiput. He 

 considers this posterior ridge to be the homologue of the post- 

 tympanic process (pars serrialis) of the squamosal in Toxodon, an 

 identification which is unquestionably correct. Exception must be 

 taken, however, to the statement that this post-tympanic process 

 is a ridge from the ventral wall of the meatus.^ The process is in 

 contact ventrally with the tympanic in the vicinity of the vagina 

 processus hyoidei but passes upwards behind the meatus in P. 

 pseudopachygnathum, and indeed in every notoungulate examined 

 by me. It seems unlikely that T. cristatum would be distinct from 

 the rest of the order in this respect. 



1 An abbreviation for the cumbersome term "crest on the under surface of the 

 tubular meatus." I previously (1932, p. 8) employed "tympanic crest" as an alter- 

 native, but this term is liable to be misleading as its Latin equivalent, crista tym- 

 panica, is applied to the margin of the sulcus tympanicus. Roth (1903, p. 19) 

 applied the name processus tympanicus to this crest. This, however, is too ambig- 

 uous as there are several closely comparable terms in existence, such as processus 

 tympanicus petrosi, alisphenoidei, etc., which might lead to confusion. 



2 Roth (1903, p. 19) also held this opinion. 



