73 



for we have examples enough of different genera having the lower molars 

 alike, while the upper ones and the premolars are unlike. 



The jaw of Trogosus retains evidences of the existence of six molar teeth, 

 and there may have been another small premolar, but this cannot be ascer- 

 tained from the mutilated condition of the specimen. The first of the series 

 of six molar teeth approached so close upon the large incisor as to leave but 

 a small interval for the introduction of other teeth. 



The best preserved tooth of the molar series, the second molar, presents a 

 bilobed crown, in which the anterior lobe is the longer or least worn. The 

 triturating surface, represented in Fig. 2, Plate V, exhibits a wide tract of 

 exposed dentine with a yoke-like outline of enamel. Its fore and aft meas- 

 urement is 9£ lines. The thickness of the anterior lobe at base is 8 lines. 



In the less worn specimen of the corresponding tooth described by Profes- 

 sor Marsh, he gives the antero-posterior diameter as 10 lines; the transverse 

 diameter at the summit of the lobes of the crown as 5 and 5£ lines wide. 



The constitution of the lower molars of Anchippodus is apparently the 

 same as in Trogosus, as observed in the New Jersey tooth represented in 

 Figs. 45, 46, Plate XXX, of " The Extinct Mammalian Fauna of Dakota," &c. 

 From this it will be seen the crown is composed very nearly on the same 

 plan as that of the corresponding teeth in Anchitherium, Palgeotherium, &c. 

 It is composed of a fore and aft pair of lobes with crescentoid summits, 

 convex externally and with a recess internally. The size of the tooth is the 

 same as that retained in the jaw-specimen above described. The fangs of 

 the last molar in the lower jaw indicate a trilobed crown, as in Anchitherium, 

 Palseotherium, &c. The premolars, so far as can be ascertained by their 

 remains in the jaw, are inserted by two fangs. Canines most probably do 

 not exist in Trogosus, their absence being fully compensated by the large 

 incisors. 



The incisor teeth on both sides together are four in number; but while 

 the lateral ones are developed to the proportions of those of rodents, the 

 intermediate pair were quite small. The latter are lost from the specimen, 

 leaving the alveoli occupied by matrix. The space they occupied was about 

 the fourth of an inch from side to side. 



The large lateral incisors are wonderfully like the incisors of rodents, not 

 only in form, position, and structure, but they were also alike in their perpet- 



10g 



