CHAPTER IX 



ISOTEMNIDAE 



THIS family is distinguished by the formula -f-j-J-f-, 

 by the incisors, canine and premolar I all being of subequal 

 size, by all the teeth being brachydont, and by the cres- 

 cents of the lower premolars and molars being modified. 

 On these lower premolars and molars the anterior crescent 

 is longer than the posterior, and the short posterior cres- 

 cent on the exterior of the tooth; so that its anterior horn, 

 instead of uniting with the posterior horn of the anterior 

 crescent, comes in back to about the middle of the anterior 

 crescent. Then the pillar, which in the other families is 

 situated in the posterior crescent, is opposite the posterior 

 horn of the posterior crescent. The small animals which 

 represent this family are rare in the Deseado beds, much 

 more abundant in the Casamayor. The family seems to 

 have died out in the Deseado as no forms are referred 

 to it in later epochs. We found no specimens belonging to 

 the family; but to make this discussion complete, I will 

 give a digest of Ameghino's descriptions, with reproductions 

 of such figures as he has given. All of the genera and 

 species are based on very fragmentary material. 



The genera assigned to the family are Trimerostephanos, 

 Pleurocoelodon, Lophocoelus and Henricofilholia. 



Trimerostephanos Ameghino 



Trimerostephanos Amegh., 1895, Bol. Inst. Geog. Argen., t. 15, p. 646. 

 Trimerostephanos Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 483. 



This genus is based on upper and lower teeth, and dis- 

 tinguished by the premolars and molars having a weak 

 style on the anterior corner, and by the anterior lobe being 

 considerably larger than the posterior. Four species have 

 been described. 



