94 
ICHTHY0D0RULITE3. 
I. Slender elongated spines, bilaterally symmetrical; the inserted 
portion smooth and usually sharply separated from the orna¬ 
mented exserted portion; internal cavity open posteriorly 
towards the base. Resembling the dorsal fin-spines of the 
Cestraciontidse, and probably for the most part referable to 
that family and to the Cochliodontidae. 
Genus ONCHUS, Agassiz. 
[Poiss. Foss. vol. iii. 1837, p. 6.J / ' '■*?- 
Syn. Archceacanthus, G. Kade, Programm. k. Realschule zu Meseritz, 
1858, p. 19. 
Spines of small size, laterally compressed ; sides of exserted por¬ 
tion ornamented with smooth or faintly crenulated longitudinal 
ridges ; no posterior denticles. 
Onchus murchisoni, Agassiz. 
1837. Onchus murchisoni, L. Agassiz, Poiss. Foss. vol. iii. p. 6, pi. i. 
fig. 1 ( ?non fiu-. 2).** 
1839. Onchus murchisoni , L. Agassiz, in Murchison’s Silur. Syst. p.607, 
pi. iv. figs. 9, 11 ( ? nonfijr. 10 i.^ 
1853. Leptocheles murchisoni, F. M‘Coy, Quart. Journ. Geol. Soc. 
vol. ix. p. 14. 
1853. Onchus murchisoni, J. W. Salter, in Murchison, Quart. Journ. 
Geol. Soc. vol. ix. p. 16. 
Type. Olim Murchison Collection. 
The type species, regarded by M‘Coy as founded upon fragments 
of Crustacean appendages, but stated by Salter to be undoubtedly 
based in part upon fish-spines. The type specimens are unfortu- 
natelyl o M 1 , ’but the spines enumerated below exhibit the characters 
described by Agassiz. 
Form. Loc. Upper Ludlow: Herefordshire, Shropshire, and 
Radnorshire. Lower Old Red Sandstone : Herefordshire 2 . 
42250 - 1 . Two fragments : Upper Ludlow, Linley Brook. 
Baugh Coll. 
1 Murchison, Quart. Journ. Geol. Soc. vol. ix. 1853, p. 16. 
2 Doubtful specimens fi*om the Upper Silurian of the Isle of Oesel, and the 
Devonian of N.W. Russia, are also assigned to this species by C. H. Pander, 
Foss. Fiscks Silui. Syst. (1856) p. 70, pi. iv. fig. 20, pi. vi. figs. 26, 27, and E. 
von Eichwald, Leth. Rossica, vol. i. (1860), p. 1591. A fragmentary spine 
much resembling this species, from Ofvedskloster, Scania, is also preserved 
in the State Museum, Stockholm. fr. /ft. /* 
fff %.**/.*. 
