426 PROCEEDINGS OF THE ACADEMY OP [1892. 



no longer deserves the name of sectorial. Among the Miistelid(e, we 

 find the upper sectorial very variously developed. In most cases it 

 is constructed as in the dogs. In the otters the deuterocone is 

 enormously expanded, both transversely and antero-posteriorly. 

 In Meles the sectorial is small, but possesses a minute tetartocone, 

 situated midway between the trito- and deuterocones, while in 

 Taxidea the tooth is large, and the tetartocone acute and promi- 

 nent and yet the crown does not lose its triangular shape. 



In the Procyonidce the upper sectorial has likewise become 

 degraded to a tubercular condition, very similar to the construction 

 of the molars, but even more complex. In Procyon there are three 

 external cusps, the anterior accessory cone, the proto- and tritocones, 

 the two latter of nearly equal height and conical shape. The inner 

 side of the crown is nearly as long antero-posteriorly, as the outer, 

 and is composed of two elements, the deutero- and tetartocones. In 

 Cercolejytes, Pj, is very similar to that of the viverrine genus Ardic- 

 tis and is a quadrituberculate tooth of nearly square outline. 



The lower premolars of the Carnivora are always more or less 

 simple and trenchant, though they vary much in respect to thickness, 

 height, acuteness, etc. They very generally display the metaconid, 

 and sometimes an additional basal cusp is formed by the elevation 

 of the cingulum posterior to the metaconid. The paraconid is 

 seldom developed, but occurs in many of the viverrines; even the 

 excessively microdont dentition of Eitpleres exhibits it. 



V. RODENTIA. 



Our knowledge of the phylogeny of the rodents is so incomplete 

 and fragmentary that little can be said with regard to the evolution 

 of their premolars. The most striking fact about these teeth is the 

 great reduction which they have undergone. In several genera 

 they are altogether absent and in the great majority of species the 

 formula is x or ^. The most ancient genus of rodents whose denti- 

 tion is Avell known is Plesiardomys which is found in all the Ameri- 

 can Eocenes above the Puerco and in the upper Eocene of Europe. 

 In those animals the premolar formula and the construction of the 

 clowns are like those of the squirrels, f- In the upper jaw, the first 

 premolar (p^) is a rudimentary tooth, and the second (P*) is molari- 

 form ; a tritubercular crown, consisting of two external cusps, the 

 proto- and tritocones, and the deuterocone internally. It is inter- 

 esting to observe that this construction of P* corresponds closely to 

 what we find in many insectivores and creodonts. The inferior pre- 

 molar also has all the elements of a molar, though their relative 



