44 ON THE EXTINCT MAMMALIA OF 



thickest and longest in front, and becomes narrowed and shortened posteriorly. Its 

 fore part is constituted of a pyramidal cusp; its back part of a broad wedge-like lobe, 

 curving outwardly. The cusp and lobe are continuous excepting at the cutting 

 margin of the tooth, where they are separated by a narrow indentation. From the 

 sharp apex of the cusp a trenchant edge sweeps in a curve to the posterior obtuse 

 termination of the crown. Antero-internally the crown forms a buttress-like column, 

 sustaining the cusp and supported by the antero-internal fang of the tooth. The 

 front surface of the crown forms an isosceles triangle, is depressed at the middle, and 

 presents below a narrow oblique basal ridge. 



The upper sectorial molar of Hycenodon horridus was worn away in a bevelled 

 manner internall}^, the cusp more rapidly than the back portion, and in such a 

 manner that the tooth always retained the form of a powerful gouge. 



As previously intimated, the construction of the alveolar border of the upper jaw 

 oi Eymnodon horridus is such that no space is left behind the position of the sectorial 

 molar for the lodgement of a tubercular molar. 



The inferior true molars, as in the case of the upper ones, likewise consist of an 

 anterior smaller pair, and a large sectorial tooth. 



The anterior pair of lower true molars, from their comparatively small size, appear 

 out of their natural position, which appearance is favored by the enamel of these 

 teeth being smoother and thinner than in any other of the molars. 



The first lower true molar is not only smaller than the succeeding teeth, but is 

 much smaller than the last premolar. Its crown is trilobate, with the median 

 conical lobe longest, the anterior lobe second in size, and with the posterior heel-like 

 lobe subtrenchant at the summit. 



The second lower true molar, less in size than the last premolar, has a trilobate 

 crown, resembling in its construction that of the lower sectorial molar of the Hyaena. 

 The anterior pair of lobes also have the form and relationship of the corresponding 

 lobes in the lower sectorial molar of the Wolf. The third is a heel-like lobe, and 

 resembles that of the tooth in advance. 



The lower sectorial molar, or third true molar of Hycenodon Jiorridus, resembles the 

 corresponding tooth of the Cats, but in its relation with the size of the skull is 

 larger and much broader in proportion with its height. The crown is composed of a 

 pair of broad wedge-like, trenchant lobes, of which the posterior is much wider than 

 the anterior. The latter is, however, the thicker, and is strengthened externally 

 towards the base by an upright buttress-like ridge. 



The inferior true molars were worn away from the apices and trenchant borders of 

 their cusps and lobes in a bevelled or sloping manner externally. 



The inner side of the first upper true molar wore a grooved surface externally upon 

 the back part of the last lower premolar and the fore part of the first lower true 



