470 GEOLOGICAL SURVEY OF THE TEEEITOEIES. 



As compared witli the Herpetotherium fwgax, this species has a shal- 

 lower mandibular bone and a larger anterior true molar. The sectorial 

 character of the oblique yoke connecting the anterior inner and outer 

 tubercles and the crescentic character of the posterior outer are not 

 nearly so well marked in the ff. fugax. 



One specimen. 



DoMNiNA CRASSiGBNis, Cope. Miotlicn crassigenis, Synopsis New Ver- 



tebrata Colorado, p. 8. 



This species is less robust than the last, and the last molar still more 

 reduced. The cusps of the latter, though not well pronounced in the 

 specimens, are homologous with those in i>. gradata. 



Eepresented by two imperfect mandibular rami, with the posterior 

 molars preserved. Last molar longitudinal and diamoned-shaped, half 

 the size of the penultimate. The latter composed of two exterior cres- 

 cents ; the cusps of the inner side worn down in both specimens, if they 

 have existed. Eamus of the mandible deep. 



Measurements. 



M. 



Lengtli of last two molars 0.0022 



Length of third molar - 0018 



Depth of ramus at third molar 0030 



DoMNiNA GEACiLis, Cope. MiotJien gracile, Cope, Synopsis Yert. 



Col., 1873, p. 8. 



This species is quite distinct from those that precede, and may, in 

 future, require the use of the generic name I formerly applied to it. It 

 is represented by a portion of a mandibular ramus, which is of slender 

 proportions. The posterior external cusps have the crescentic form 

 characteristic of this genus. 



Last molar nearly as large as penultimate, with a low cusp at each 

 extremity and an emarginate cross-crest at the middle. Penultimate 

 molar with three inner cusps and two outer crescents. Eamus of man- 

 dible slender. 



Measure7nents. 



M. 



Length of last two molars &.0025 



Length of penultimate molar 0012 



Elevation of penultimate molar 0009 



Depth "of ramus at penultimate molar 0021 



ISACIS, Cope. 



Paleontological Bulletin, (August, 1873,) No. 16, p. 3.— Bulletin U. S. Geol. Survey 

 ^ Terrs., No. 1, 1874, p. 23. 



This genus embraces at present but a single species, which is known 

 from numerous specimens discovered by the writer in the Miocene for- 

 mation of Colorado. From these it. appears that Isacis is closely allied 

 to the Leptictis and Ictops of Leidy, occupying a position between them 

 in the system. In Leptictis the last premolar is sectorial in form, con- 

 sisting of a single compressed longitudinal crest, without internal tu- 

 berosity or cusp. In Ictops, the last premolar exhibits a structure similar 

 to that of the first true molar, viz, two exterior cusps, and well devel- 

 oped third on their inner side, thus giving a horizontal section of the 

 tooth a subtriangular form. In Isacis, the last premolar possesses a single 

 acute cusp, as in Leptictis, with an internal cusp or heel^ homologous 



