1892.] NATURAL SCIENCES OF PHILADELPHIA. 425 



The premolars of the fissipede carnivores have a different history, 

 lu the upper jaw the anterior premolars are ordinarily composed of a 

 compressed and more or less trenchant protocone, and on Pz and 3 a 

 small tritocone may also be added. According to Schlosser (No. 10 

 p. 265) the Galeeynns ceiiingensis of Owen is remarkable for the 

 presence of anterior cusps upon tbese teeth. The fourth or sectorial 

 premolar is constructed on essentially the same plan throughout the 

 group, a plan which is already foreshadowed in many creodonts 

 and fully attained in the Mlacidce. Typically this tooth consists of 

 three elements, the proto-, deutero- and tritocones, the first and 

 third of which are more or less compressed and trenchant and form 

 the shearing blade. But there is much variation in regard to the 

 relative size and shape of these elements, and consequently in the 

 efficiency of the tooth as a sectorial weapon. In several groups a 

 fourth element is added ; viz., a cusp situated at the anteroexternal 

 angle of the crown. This cusp, which appears to be homologous 

 with the antero-external buttress or pillar of the perissodactyl 

 premolar reaches its highest development in the cats, though in the 

 earliest members of that group, the Nimravidce, it is always small 

 and sometimes rudimentary or absent (e. g. Dinictis.) The same 

 element occurs in the viverrines, the hysenas, and the curious cy- 

 noid genus JElurodon of the Loup Fork has it largely developed. 

 The dentition of this genus presents some analogies with that of the 

 hytenas, especially in the massiveness of the premolars. The same 

 cusp also occurs in the creodont Didymictis. In the Viverridce the 

 deuterocone is unusually well developed, and in some of the genera 

 is joined to the protocone by a sharp ridge running outward and 

 backward. The hyseuas display an unusual complication in that 

 the outer wall of the sectorial consists of four cusps, an additional 

 one occurring behind the tritocone. 



The derivation of the bears fi-om primitive dogs is now very 

 generally admitted and the series of genera, Amphicyon, Bemicyon, 

 Hycenarctos and JJrsus, enables us to trace the successive modifica- 

 tions of the upper sectorial. In the first named genus, the upper 

 sectorial is like that of a dog, but somewhat thickened ; in Hemi- 

 cyon, the tooth has become transversely extended and the deutero- 

 cone has shifted from the anterior edge to the middle line of the 

 crown. In Hycenarctos, P* is reduced in relative size, and all the 

 cusps are lower and more massive, and in Ursiis, the same process is 

 carried still further; the deuterocone has become almost atrophied, 

 and the proto- and tritocones are so low and blunt that the tooth 



