Pisczs. | LOWER PALAZOZOIC VERTEBRATA. 577 
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 cartilaginous fishes; and finally, having made an examination of transparent 
sections in a powerful miseroscope, 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 per- 
form.” On the day the above paper was read to the Society, the late Prof. Strickland also brought up 
a paper on the Ludlow bone-bed, in which he had independently suggested (from their immense profusion 
and small size) that these little objects were not teeth, but shagreen granules, without having however proved 
it structurally as I had done. 
Position and Locality—Occurs in immense numbers in the bone-bed of the upper Ludlow rock near 
Downton Castle, forming masses some inches thick, and extending for miles; also in the bone-bed, Ludford, 
near Ludlow. 
Genus. ONCHUS (Ag.) 
Gen. Char.—Fin-ray compressed, tapering, gently arched backwards towards the apex; anterior face 
obtusely keeled, posterior face destitute of rows of denticles; surface longitudinally sulcated ; numerous inter- 
mediate small ridges smooth. Jase of insertion small, longitudinally striated, rapidly tapering. 
The absence of the two rows of denticles on the posterior face separate these fin-defences from those of 
the genus Hybodus ; the same character and smoothness of the lateral sulci separate them from Ctenacanthus. 
All the species are Palzeozoic. 
ONCHUS TENUISTRIATUS (Ag.) 
Ref—Murchison, Sil. Syst. t. 4. f. 57 to 59. 
Dese.—Usual length about one inch nine lines along the front from base to apex, which is rather abruptly 
tapered, and curved backwards more or less strongly ; the anterior face more curved than the posterior ; width 
near base slightly exceeding two lines ; posterior face apparently smooth, slightly concave ; anterior face obtusely 
angulated, rounded, the sides of the undefined keel forming a smooth space on each side, equal in width to the 
two next ribs ; longitudinal ridges very unequal, smooth, flattened, separated by deep narrower sulci (five ridges 
in the width of two lines at about the middle of the spine, and they vary irregularly at any given length from 
one-half to one-fourth of a line in width) ; covered part of base short, abruptly tapered. 
As well as I can make out, there are about eight longitudinal ridges on each side of the widest part of the 
spine, and they, of course, diminish in number towards the apex; they are remarkably unequal in size, and irre- 
gular in the relative positions of the small and large ones. The substance is very thick, with a moderate internal 
cavity. I have examined portions prepared as transparent microscopic objects, under a high magnifying power, 
and find the characteristic structure and radiated cells of true bone distinctly marked. 
Position and Locality Common, mingled with what I conceive to have been the grains of their shagreen 
(Thelodus parvidens Ag.), in the bone-bed of the Upper Ludlow rock at Ludford, near Ludlow. 
