154 
SELACHII. 
Xiphotrygon acutidens, Cope. 
1877. Heliobatis radians, 0. C. Marsh, Arner. Journ. Sci. [3] vol. xiv. 
p. 256 (undefined). 
1879. Xiphotrygon acutidens, E. D. Cope, Amer. Nat. p. 333. 
1884. Xiphotrygon acutidens, E. B. Cope, Vert. Tert. Form. West, 
Book I. (Rep. U. S. Geol. Surv. Territ. vol. iii.) ; p. 50, pi. i. 
figs. 1, 5. 
Type. Complete skeleton. 
Bisk longer than broad, and pectoral fins prolonged anteriorly 
into a pointed median projection. Caudal spines, triangular in 
section, three. 
Form. <$f Loc. Middle Eocene (Green River Shales) : Twin Creek, 
S.W. Wyoming, U.S.A. 
Genus UROLOPHUS, Muller & Henle. 
[Syst. Beschreib. Plagiost. 1841, p. 173.] 
Tail of moderate length, with a distinct rayed terminal fin and a 
barbed spine : rudimentary dorsal fin sometimes present. Mouth 
and dentition as in Trygon. 
Urolophus princeps, Heckel. 
1854. Urolophus princeps, J. J. Heckel, Sitzungsb. math.-nat. Cl. k. Ak. 
Wiss. Wien, vol. xi. p. 124. 
1863. Urolophus princeps, R. Kner & F. Steindachner, Denkschr. k. 
Akad. Wiss. Wien, vol. xxi. p. 32, pi. vi. fig. 2. 
Type . Complete skeleton ; Hof-Museum, Yienna. 
Bisk broader than long; snout apparently not projecting. Tail 
shorter than disk; caudal spine extremely large ; terminal rayed 
fin well developed. Skin with small granular calcifications, not 
spinous. 
Form. Sf Loc. Middle Eocene : Monte Postale, N. Italy. 
A vertebra from the Middle Eocene (Bruxellian) of Etterbeck, 
Belgium, is referred to an undetermined species of Urolophus by C. 
Hasse, Natiirl. Syst. Elasm., Besond. Theil, p. 147, pi. xix. figs. 10- 
14. Others, from beds of corresponding age in Samian d, East 
Prussia, are described as Urolophus (?) bicuneatus by E. Noetling, 
Abb. Geol. Specialk. Preussen u. Thiiring. Staaten, vol. vi. pt. 3 
(1885), p. 34, pi. vii. fig. 8, pi. viii. fig. 11. 
