﻿94 [CHTHTODORXrUTEB. 



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 

 (Vstraciontidae, and probably for the most part referable to 

 that family and to the Cochliodontidas. 



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 fig. 2). 

 ]839. Onchus murchisoni, L. Agassiz, in Murchison's Silur. Syst. p. 607, 



pi. iv. figs. 9, 11 (? non fig. 10). 

 y 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- 

 nately lost \ but the spines enumerated below exhibit the characters 

 described by Agassiz. 



Form. Sf 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 from 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. Fische Silui. Syst. (1856) p. 70, pi. iv. fig. 20, pi. vi. figs. 26, 27, and E. 

 Ton Eichwald, Leth. Rossica, vol. i. (1860^, p. 1594. A fragmentary spine 

 much resembling this species, from Ofvedskloster, Scania, is also preserved 

 in the State Museum, Stockholm. 



