LAMELLIBRANCHIATA., | UPPER PAL/EOZOIC MOLLUSCA. 481 
This species is distinguished from the P. sulcatus (M*Coy) and the P. semisulcatus (M°Coy) (of which 
latter it has the exact form) by having all the posterior part of the shell striated ; in its ridging it agrees with 
the Lanistes rugosus (M*Coy, Synop. t. 10. f. 8), but the oblong form, and broad beak and anterior end seem 
to separate the latter. It grows larger than the above measurements. The adductor impression is moderately 
large, rounded, placed a little in front of the middle of the posterior slope, partly on the posterior wing, and 
partly on the body of the shell. 
Position and Locality—Not uncommon both in the main limestone of Derbyshire and the black limestone 
resting on it; also in the impure limestone of Lowick, Northumberland. 
Explanation of Figures—Pl. 3. F. Fig. 1. Right valve, natural size: from the carboniferous limestone of 
Lowick, Northumberland. 
Genus. AUCELLA (Keyserling). 1846. 
Gen. Char.—Obliquely ovate, very inequivalve, and inequilateral ; left valve very convex, with a large beak 
spirally inrolled towards the anterior side; hinge-line forming a short, flattened, obtuse wing on the posterior 
side; anterior wing almost obsolete, not defined from the body of the shell by any byssiferous sinus ; right valve 
flat or slightly convex, with a small distinct triangular ear, as in Pecten, separated from the body of the shell 
by a deep, narrow, byssiferous notch ; hinge edentulus and without cartilage facet ; adductor impression single, 
rounded, a little behind and above the middle; several small retractor impressions near the beak. Texture of 
the shell pearly within, prismatic-fibrous without. 
The great inequality of the valves and inrolled beak of the left one, &c. separate these shells from A viculo- 
pecten, while the byssiferous sinus being confined to one valve and forming in it a deep notch defining a long 
anterior ear as in Pecten, &c., distinguish them from A vicula. 
AUCELLA SPELUNCARIA (Schlot. Sp.) 
Ref. and Syn. = Gryphites spelunearius Schlot. Akad. Minch. for 1816, t. 5. f. 1. = Avicula grypheoides Sow. 
Eneye. Metrop. Vol. 1V. t. 3. f. 6. = Monotis speluncarius (Schlot.) King, Perm, Foss. t. 13. f. 5 to 21. 
Dese.—Subhemispherical, slightly oblique towards the posterior side, which is largest. Left valve very 
gibbous along the middle, greatest depth about the middle; beak very large, obtuse, much incurved ; anterior 
side extremely small, nearly rectangular, flattened; posterior wing obtusely angular, tumid, and defined from 
the posterior slope by an obtuse sulcus, sometimes obsolete near the beak ; anterior and ventral margins semi- 
circularly curved to the small sinus at the junction of posterior wing, the margin of which is moderately convex ; 
surface of old specimens nearly smooth towards the margin, strongly marked for about six or eight lines from 
the beak by narrow, filiform, slightly irregular ridges, either subequal and separated by slightly wider spaces, 
or having either one slightly smaller ridge, or three striz between each larger pair, often rendered subspinulose 
by scale-like projections of the imbricating lines of growth (averaging seven of the larger striz in two lines at 
six lines from the beak). Right valve flat, suborbicular; the posterior wing defined by a slight ridge from the 
beak to a minute sinus in the margin, corresponding with the sulcus on the opposite valve ; anterior ear narrow, 
triangular, completely separated up to the beak from the body of the shell by a deep triangular byssiferous notch ; 
surface having the concentric strize stronger, and the radiating striz usually fainter than the opposite valve. 
20 
Average width from beak to middle of ventral margin one inch three lines, proportional length of anterior ear =, 
length of posterior side , width of posterior wing from sulcus =, depth of left valve =. 
This species varies considerably in the sculpturing of the surface, most of the specimens seeming nearly 
smooth, except towards the beaks, others having the longitudinal ridging remarkably strong, but varying in 
their relative size, while others are remarkably distinguished by the vaulted scale-like, or subspinulose character 
given by the concentric, imbricating striz. Although Count Keyserling does not place this in his genus Aucedla, 
still from the accordance of all those characters which can be seen by the naked eye, I have little hesitation in 
placing it there. 
Position and Locality—V ery common in the Permian limestone of Humbleton. 
3Q2 
