Bracutopopa.| DEVONIAN MOLLUSCA. 385 
This species varies considerably in proportional width, but principally owing to pressure. The minute 
interstitial ridge, from which the species derives its name, and in some rare cases three minute ridges, are deve- 
loped between a pair of larger near the margin ; in impressions of the outer surface these of course appear as 
depressed lines dividing the ridges. On examining the type specimen of Mr Sowerby’s Atrypa striatula, 1 find 
that species is founded on an Orthis, an internal cast of a large specimen of the O. interlineata of the same writer, 
shewing on careful examination the same interlined ridging, same number of ridges in a given space, &c.; in 
addition to which I myself have collected all the intermediate sizes in the original locality of both species at 
S. Petherwin. The internal casts alluded to by Mr Sowerby as Afrypa striatula from Fowey, is generically 
distinct, and probably Spirigerina desquamata. 
Position and Locality.—Extremely abundant in the slate of S. Petherwin, and Lanlake, Launceston. 
ORTHIS PERSARMENTOSA (A/‘Coy). PI. 2. A. fig. 9. 
Sp. Ch.—Transversely oblong, hinge-line nearly as wide as the shell, ends obtusely subtruncate, slightly 
rounded: surface covered with thick, rugged, rounded, flexuous, radiating ridges, about half their thickness 
apart ; about five in three lines in the middle of the shell at six lines from the beak, branching four or five times 
between the beak and margin, counting about 130 at the margin of a large specimen; those towards the sides 
straighter and finer than those in the middle. Average width three inches, length probably one-third of the 
width, but cannot be stated accurately owing to the distortion of all the specimens. 
This species very closely resembles the O. sarmentosa (M*Coy) of the older rocks in form, and the peculiar 
twig-like mode of branching of the ribs on the middle of tie shell, and the straighter and finer ones of the sides, 
but is distinguished by the very much greater number of the ridges As in the case of that species, the distortion 
is usually such, that I ean make no probable approximation to the proportional length. The coarseness of the 
ridging separates the present species from the American Strophomena nervosa and S. bifurcata (Hall) of the 
Chemung group. 
Position and Locality—Common in the Devonian shale of Polruan, Cornwall; in the reddish Devonian 
schists of East Looe; schists of Fowey. 
Explanation of Figures.—PI. 2. A. fig. 9, from the Devonian shales of Polruan, Cornwall; natural size 
of a specimen shortened by crushing. 
> 
Genus. LEPTAINA. See page 232. 
Subgenus. STROPHOMENA. See page 241. 
LepTaNnaA (Strophomena) ARACHNOIDEA (Phill. Sp.) 
Ref—Spirifer id. Phill. Geol. York, Vol. II. t. 11. f. 4. 
Sp. Ch.—Truncato-orbicular, sides subparallel, slightly convex, front broadly rounded, slightly convex, 
much flattened; hinge-line as wide as the shell, cardinal area very narrow ; ears rectangular; entering valve 
slightly convex, with a straight hinge-line ; receiving valve with a narrow moderately prominent beak ; rostral 
portion slightly convex, becoming flattened or very slightly concave towards the margins; surface of both valves 
radiated with very numerous, narrow, thread-like ridges, separated by wider, flat, or slightly concave spaces, 
crossed by minute lines of growth under the lens; the ridges increase very much in number (but not in 
thickness) between the beak and the margin by intercalation of one ridge at a time, which quickly assumes the 
thickness of the original pair between which it commenced, so that the strize are all nearly equal at the margin, 
or rarely a single smaller between some of the pairs of ordinary size; nine or ten strize in two lines at eight lines 
from the beak, about the same at six lines from the beak; in the entering valve the lateral striz are nearly 
straight along the hinge-line, but in the receiving valve they are slightly arched divaricatingly. Average 
width one inch two lines, proportional length 5. 
[Fasc. 11] 3D 
