002 INDEX GENERUM MAMMALIUM. 
Ischyromys Lripy, 1856. Glires, Ischyromyide. 
Proc. Acad. Nat. Sci. Phila., 1856, 89. 
Type: Ischyromys typus Leidy, from the Oligocene of the Bad Lands of ‘ Nebraska’ 
(or South Dakota?). 
Extinct. Based on ‘‘the greater portion of a skull and two fragments of lower 
jaws." - 
Ischyromys: @6xvpos, strong; “0s, mouse. 
Ischyrorhynchus AmeGcurno, 1891. Cete, Platanistidee. 
Revista Argentina Hist. Nat., I, entr. 3a, 163-165, figs. 71, 72, June 1, 1891. 
Type: Ischyrorhynchus vanbenedeni Ameghino, from the Lower Eocene of 
Paraná, Argentina. 
Extinct. 
Ischyrorhynchus: @6xvpos, strong; ovy xos, snout. 
[Ischyrotherium Lespy, 1856. Reptilia. 
Proc. Acad. Nat. Sci. Phila., 89, 1856. 
Type: Ischyrotherium antiquus Leidy, from a lignite formation between Moreau 
and Grand Rivers, South Dakota? * 
Originally described as a cetacean. 
Extinct. Based on ‘numerous fragments of bones.’ 
Ischyrotherium: @6yvpos, hard; 65píov, wild beast—in allusion to the fact that 
**the bones are as dense and heavy as those of Manatus.”’ ] 
Isectolophus Scorr & OsBorn, 1887. Ungulata, Perissodactyla, Tapiride. 
Proc. Am. Philos. Soc., XXIV, No. 126, pp. 260-261, Nov. 2, 1887; OsBorn, Trans. 
Am. Philos. Soc., new ser., X VI, pt. 111, 518-524, pl. x, figs. 1-8, Aug. 20, 1889. 
Type: Isectolophus annectens Scott & Osborn, from the Uinta Eocene of White 
River, northeastern Utah. 
Extinct. Based on ‘‘the second premolar and first and second molars of the 
maxillary series, and the last lower molar and portions of the last premolar 
and first molar of the mandibular series.”’ 
Isectolophus: 160, equal; é«rds, outside; Add@os, crest—in allusion to the 
external cusps (paracone and metacone) of the upper molars, which are equal 
in size, in contrast with those of Helaletes. 
Isocetus VAN BENEDEN, 1880. Cete, Balzenidee. 
Bull. Acad. Roy. Sci. Belgique, 2* sér., L, 24-25, 1880. 
Type: Jsocetus depawwii Van Beneden, from the vicinity of Antwerp, Belgium. 
Extinct. *'Outre le fragment de crane, les caisses tympaniques, nous en avons 
une mandibule, une région cervicale, des vertébres dorsales, des cótes et des os 
de membres." 
Isocetus: 760s, equal; «?jroc, whale. 
Isodelta (subgenus of Arvicola) Corr, 1871. Glires, Muridee, Microtinze. 
Proc. Am. Philos. Soc., XII, 87-88, fig. 13, Jan.-July, 1871; Journ. Acad. Nat. 
Sci. Phila., 2d ser., XI, pt. 2, pp. 205, 206, 1899. 
Type: Arvicola speothen Cope, from the Pleistocene of the Port Kennedy Bone 
Cave, Montgomery County, Pennsylvania. 
Extinct. Based on *'the entire dentition of the left ramus mandibuli, with a 
few fragments of the adjacent bone." 
Isodelta: 7605, equal; 8£Ara, the Greek letter 4, a triangle—in allusion to the 
equality of the triangles of the second lower molar. 

* Marsh states (Am. Journ. Sci., 3d ser., XX XVIII, 81, July, 1889) that the type 
of Ischyrotherium antiquus came from Judith Basin, Montana, and that the remains 
are those of a reptile, as shown by Cope (Syn. Ext. Batr. Rept., and Aves N. Am., 
38, 1869). 
