1<)10.] Mammal-like Characters of Cynodonts. 121 



The basioccipital region in Cynodonts is extremely short antero-j)os- 

 teriovly. a fact probably im])lying a very small brain. Tlie occipital condyle 

 is bilaterally paired and although still continuous across the basioccipital 

 is quite prophetic of the mammalian condition, as observed by Seeley (1895.3, 

 p. 1). The exoccipitals are broad laterally-expanding wings, but, as in 

 certain Insectivores and other mammals, are not produced downward into 

 paroccipital processes. Immediately back of the glenoid region is a groove 

 in the position of the osseous external auditory meatus of mammals (Fig. 1, 

 B, m. a. c.) which is apparently homologous with that structure. At any rate 

 it is, as in the mammals, bounded anteriorly by the postglenoid region, pos- 

 teriorly by an apparent post-tympanic process, and superiorly by a ridge 

 connecting the superior border of the zygomatic arch with the lambdoidal 

 crest. 



Running inward from the articular region is a horizontal bar (Fig. 1, 

 B, fty.) of somewhat doubtful homology. Broom (1904.2, p. 491) holds that 

 this bone is the tympanic. He thinks that it cannot be a "rudimentary 

 straight cochlea" (Seeley, 1895.4, p. 138), because in Oudcnodon "the 

 bone is solid, so that it cannot have lodged any part of the inner ear." It 

 cannot be the columella auris itself because in Dicijnodon the columella aiu'is 

 lies above the bone in question, "in the hollow formed by the bone and the 

 exoccipital." In the skull of Trirachodon kannemeyeri, Broom (/. c, p. 493) 

 found a "delicate bony rod of about the thickness of a pin, which exactly 

 corresponds with the tympanic in Ci/)wgnathus." The homology of this 

 important bone must be definitely settled if the history of the mammalian 

 ossicula auditus is ever to be elucidated. 



Lower jau\ — In the relations of the lower jaw and its component parts 

 to the skull the typical Cynodonts ^ (Fig. 2, B) had advanced much beyond 

 the Therocephalians (Fig. 2, A) in the further reduction of the quadrate 

 and in the development of the dentary, which is now the predominant ele- 

 ment of the lower jaw (Broom, 1908, p. 1053). The surangular has become 

 reduced; the rather slender angular is conjoined with the small articular. 

 All these characters foreshadow the mammalian conditions, as will be more 

 fully shown below. 



1 Bauria is much more primitive, since it retains a large angular and surangular (Broom, 

 1909.1, p. 272). 



