77 



The posterior horn of the crescentoid summit of the postero-external lobe 

 ends in the tubercle at the back of the crown, and in the last molar, in the 

 homologous fifth lobe. The latter is joined by an acute ridge, descending to 

 the base of the postero-internal.lobe. 



A thin basal ridge exists at the fore and back parts of the crown of the 

 first and second molars, and the fore part in the last molar. An element 

 also exists at the interval, externally, of the outer principal lobes, and in some 

 specimens is more or less produced around the bottom of the antero-external 

 lobe. 



As the crowns of the true molars are worn away, circular islets of dentine 

 appear at the summits of the inner lobes, and crescentic islets at the summits 

 of the outer lobes. In the progress of attrition the dentinal surfaces expand, 

 and the horns of the crescentic islets become united with the circular islets. 

 In an advanced stage of wear the triturating surface of the molars presents two 

 elliptical surfaces crossing the crown, with a slight obliquity, and united by a 

 median isthmus, the whole bordered by a band of enamel. Such a condition 

 is seen in the specimen represented in Figs. 1, 2, Plate VI, which is that 

 upon which the genus was originally proposed. By comparing this with the 

 others in different stages of wear, represented in Figs. 3 to 9, of the same 

 plate, its correspondence with these, which preserve more characteristic 

 generic marks, can be readily recognized. 



The last lower premolar of Hyopsodus (Figs. 5, 8, Plate VI) is smaller 

 than the true molars, and like them is inserted by a pair of fangs. Its crown 

 is proportionately of greater depth than in the true molars, and, as in these, is 

 widest fore and aft. The outer fore part of the crown is composed of a demi- 

 conoidal lobe, which is the principal one, and it corresponds with the antero- 

 external lobe of the true molars. It has the same form as the latter lobe, but is 

 better developed. The anterior horn of its crescentoid summit forms a curved 

 ridge, defining the fore part of the triturating surface of the crown. The poste- 

 rior horn of the crescentoid summit terminates in a small conical lobe occupying 

 the middle of the crown internally. The back of the crown is formed by a 

 broad heel, skirted by a basal ridge externally, and divided by another ridge, 

 which descends from the summit of the principal lobe of the crown, and bor- 

 ders the heel posteriorly and internally. A thin basal ridge occupies the fore 

 part of the crown. In the wearing of the crown of the last premolar, the 

 exposed dentine assumes the form of the* Greek letter f, lying on its right 

 side. 



