1852.] M'COY SILURIAN FISH AND CRUSTACEANS. 15 



shagreen of the same fish, in all probability, of which fragments of 

 the bony dorsal rays {Onchus tenuistriatus) are so common inter- 

 mingled in the same mass. M. Agassiz, judging only from the 

 drawing (figs. 34, 35, 36), supposed these magnified and isolated 

 specimens to resemble teeth of the general character of Lepidotus ; 

 but one glance at the specimens would dissipate this notion, when we 

 find that they are square and not rounded, that they are as small as 

 grains of fine sand, and occur in such abundance over large patches 

 of rock as to resemble thick layers of sand. All these points speak 

 against their being teeth, but are in accordance with the supposition 

 of their being the earthy grains or shagreen of the skin of large carti- 

 laginous fishes ; and, finally, having made an examination of trans- 

 parent sections in a powerful microscope, I found, instead of the close 

 dentine of the teeth of Lepidotus, only the loose divaricating tubular 

 structure usually found in such dermal armature as has no grinding 

 duty to perform. 



Postscript. — The specimens forwarded to me, from the Collection 

 of the Geological Society, as supposed Ichthyodorulites from the 

 Wenlock limestone of Whitfield, Tortworth, are the nacreous, 

 shelly tubes of a new species of Serpulites, which I name S. perver- 

 sus from the singular character of the general curve of the fossil 

 being in a plane at right angles to that in which the tube is com- 

 pressed. The average length is 3 inches, and width 2\ lines at the 

 large end. 



Serpulites perversus {M'Coy), from Whitfield Quarry, near Tort- 

 worth, Gloucestershire. Collected by T. Weaver, Esq., F.G.S. 



a. Side view of a large specimen ; natural size. 



b. Section at the thickest end. 



