474 DE. H. W. MAEETT TIMS ON THE 



next structure to whicli I attach great importance is tlie internal 

 cingulum. "We have already noted its constant presence. That 

 it is a structure of great antiquity is proved both embryologically 

 and palseontologically. If the teeth of a foetal pup be examined 

 the cingulum is, proportionately to the primary cone, much 

 larger than in the fully-developed tooth. It is also present in 

 some of the fossil Mammalia of the Stonesfield Slate (5) ; as, for 

 example, Amphitherium. The ends of this internal cingulum, 

 vrhich is generally regarded as a mammalian characteristic, give 

 rise to the small anterior and posterior cusps, such as we have 

 seen exist in the incisor teeth of the Dog. Such a form of tooth 

 at once suggests that of the fossil Microconodon. In the more 

 specialized cheek-teeth the cingulum, though always more marked 

 internally, is continued round the external face of the tooth, and, 

 as we have seen, may give rise to cusps externally, as in Otocyon 

 and possibly some of the Insectivora. 



This description of a tooth with a main cone and small anterior 

 and posterior cusps agrees with the description of the premolars 

 of AmpJiitJierium Prevostii given by Owen (18). These teeth, 

 he says, " consist oE a single compressed conical cusp with a 

 minute tubercle at the hind part of its base and a more minute 

 one in front." We have already seen that in the teeth of the 

 Dog the posterior cingulum-cusp is usually more marked than 

 the anterior. Groodrich (5), in describing the British Museum 

 specimen of A. Prevostii, which he has fully exposed, says (p. 414) 

 that the premolars have " a laterally compressed crown bearing 

 one large cusp, a very small anterior cingulum-cusp, and a 

 posterior heel." 



The anterior cingulum-cusp does not usually undergo further 

 development. In certain cases, however, in which it does do so, 

 it may form an anterior cone, thus giving rise to a Triconodont 

 tooth. The only teeth in the Dog in which it undergoes deve- 

 lopment are the deciduous and permanent lower carnassials, 

 giving rise to the cusp usually termed the Paraconid. In these 

 teeth the cingulum runs right up into that cusp, and does not 

 extend forwards as is the case at the posterior end of the tooth. 

 This anterior cingulum-cusp is placed somewhat to the inner 

 side of the primary cone, giving rise to the Protocone of the 

 upper premolars of the Dog and the Paraconid of the lower 

 carnassial. 



The posterior cingulum-cusp is usually well-marked, and in 



