G. I. Adams — Extinct Felidce of Worth America. 437 



cusp of the inferior sectorial and a modification of the heel. 

 Thus is reached the most primitive species of the genus. 

 Within the limits we can trace the reduction of the second 

 inferior molar from a small two-rooted tooth to a mere 

 tubercle and finally find it entirely absent in D. platycopis. 

 The genus Hoplophoneus is well separated from Dinictis 

 through the development of an anterior basal cusp on the 

 superior sectorial and the loss of the internal cusp. The internal 

 cusp had already shown some signs of reduction in certain 

 specimens of Dinictis in that it had become lower and less 

 distinct. Moreover, Hoplophoneus has lost the second inferior 

 premolar and within the genus we can trace the reduction and 

 final disappearance of the second superior premolar. The 

 postero-internal cusp of the inferior sectorial, which had 

 shown signs of variation in Dinictis, is absent in the higher 

 species of Hoplojphoneus. The rapidly diminishing dental 

 series reaches its maximum reduction in Eusmilus. Of this 

 only the inferior dentition is known. It differs from Hoplo- 

 phoneus occidentalis in the loss of an incisor, the reduction of 

 the heel of the sectorial in E. bidentatus and its absence in 

 E. dakotensis. This is the only genus in which the incisors 

 have suffered a reduction in number, although in some speci- 

 mens of Hoplophoneus the internal ones are much reduced. 



With the formula IfC^PmfMi the most stable form of 

 Machserodont dentition seems to have been reached, since it has 

 given rise through the addition of basal cutting lobes to genera 

 which persisted until Pleistocene times. The genus Machcero- 

 dus differs from the genus Smilodon in having a second basal 

 lobe developed on the molar series. They are further distin- 

 guished by the fact that in Smilodon the post-glenoid and post- 

 tympanic are coossified. The absence of the entepicondylar 

 foramen in the specimen of S. necator figured by Cope* can- 

 not be considered of generic importance since Burmeister has 

 shown that it is present in three specimens which he has 

 described. f Its absence in the case above mentioned is prob- 

 ably due to individual variation. The most specialized genus 

 as regards the structure of the teeth is Dinobastis. In Hoplo- 

 phoneus the internal cusp of the superior sectorial has disap- 

 peared and the root supports a convex buttress. This is also 

 the case in Machwrodus and Smilodon, but in Dinobastis even 

 the internal root has disappeared. The incisors also present a 

 peculiarity, which is seen in some specimens of Machwrodus 

 and Smilodon, in the possession of minute basal cusps and 

 crenulate margins. With this genus the Machserodonts cul- 

 minated in specialization of tooth structure as they did in 



*Amer. Nat., 1880, p. 857. 



f Description physique de la Republique Argentine, Text in Atlas, p. 106. 



Am. Jour. Sci— Fourth Series, Vol. I, No. 6.— June, 1896. 

 29 



