Bracuiopopa. | UPPER PALOZOIC MOLLUSCA., 435 
ATHYRIS GREGARIA (J/°Coy). 
Ref. and Syn. = Atrypa id. M°Coy, Carb. Foss, Irel. t. 22. f. 18. 
Dese—Longitudinally trigonal, greatest width near the front margin; lateral anterior angles rounded ; 
lateral margins with a small rounded wave towards the entering valve, thence remaining nearly in one plane till the 
middle of the front margin of specimens exceeding five lines long, in which it is raised into a very shallow, wide 
sinus ; valves nearly equal in depth, sometimes the receiving and sometimes the entering one deepest. Entering 
valye moderately convex along the middle, most tumid close to the beak in many specimens, no distinct mesial 
ridge. Receiving valve with a very large, projecting, moderately incurved thick beak, with a small foramen at 
the apex; the sides near the beak are inflected to form an obtusely-defined, elongate, flattened, or slightly 
coneave space on each side, which is also slightly marked in some specimens of the entering valve, but 
nearly absent from both valves in some specimens; the long posterior lateral margins very obtusely rounded ; 
middle of the valve, after five lines long, depressed into a very wide shallow sinus, corresponding with the 
wave in the front margin. Surface under the lens shewing fine, concentric, obtuse striz of growth, crossed 
by obsolete, obtuse, longitudinal strize, nearly equalling them in size, but very much larger than the fibrous 
tissue of the shell. Length of average specimen eight lines, greatest proportional width (near the front 
85 55 
margin) =, length of entering valve ;;, depth of both valves 5. 
Var. a. TRAPEZOIDALIS (M°Coy). P1.3. D. fig. 20*. 
Form varying from very broad-ovate, to trapezoidal; greatest width a little in front of the middle; length 
equalling or considerably exceeding the width ; lateral inflection of the margins nearly obsolete ; beak of receiving 
valve small, abruptly incurved; the produced narrowed front scarcely raised at the margin, even at seyen or 
eight lines long, and producing no distinct mesial hollow in the receiving valve. Other characters as in the 
above type. 
It is only after much hesitation that I venture to unite the Kendal and Derbyshire fossils with my 
originally-described A. gregaria; that species occurring gregariously in immense profusion in the limestone 
of Church Hill, Fermanagh, also forming great masses in that of Ballycastle, Antrim, and being remarkably 
constant in its singular trigonal form, wide front, and very obtuse, nearly straight sides and large beak: some 
of the Kendal specimens however agree exactly with the Irish ones, although the more common variety there 
has the front narrowed, and produced, and the lateral inflections of the margins near the thickened beaks are 
less marked ; these seem to pass insensibly into the ovate forms, with the small beak, a most extreme example 
of which I have figured Pl. 3. D. f. 20*. 
The trapezoidal variety with the narrow rounded front, long posterior lateral margins meeting the straight- 
ened, anterior, shorter lateral margins at the obtusely-rounded, narrowed, lateral angles, which are only a 
little in front of the middle, and in which the beak is also smaller than in the principal type, seems to abound 
most in the Derbyshire limestone, but seems to pass gradually into both the ovate and trigonal forms. The 
spiral appendages are of moderate size. When decorticated several strong longitudinal impressions are often 
seen radiating from the beak toward the front. 
Position and Locality—Gregarious in considerable abundance in the lower carboniferous limestone of 
Kendal, Westmoreland; also in the lower carboniferous limestone of Derbyshire; rare in the lower car- 
boniferous limestone of Lowick, Northumberland. 
Explanation of Figures.—P1. 3. D. fig. 20*, abnormal ovate variety, with small beak, front view, natural 
size from the limestone of Kendal; fig. 20* a, do. end view, shewing the very slight wave in the front margin ; 
fig. 20* 5, do. profile; fig. 20* ¢, surface strongly magnified, shewing the fibrous tissue, and the longitudinal 
obtuse strice. 
