262 BRITISH PALAXOZOIC FOSSILS. [ LAMELLIBRANCHIATA. 
I can scarcely doubt the identity of our shell with that figured and described (not very fully) as above by 
Mr Conrad, from the Upper Silurian shales of New York; but if any character not indicated in the American 
species should hereafter separate them, the name meso-clathrata might be applied to this one to indicate a 
character which separates it from the varieties of P. lineata or P. reticulata, namely, the radiating lines being 
confined to the middle of the shell, not affecting the wings. 
Position and Locality.—Bala schists, Cyrn y Brain, west of Wrexham, Denbighshire; the large variety 
from the Ludlow rock at Parklane, Llandeilo; one doubtful variety in the Upper Ludlow quartzite of Benson 
Knot, Kendal, Westmoreland. 
Explanation of Figures —P1. 1. I. fig. 1. Natural size, left valve, large specimen from Parklane, Llan- 
deilo; fig. 2, small variety from Cyrn y Brain; fig. 2.@, Ditto, profile of both valves, natural size ; fig. 24, part 
of surface of ditto magnified. 
PTERINEA RETROFLEXA (Wahl. Sp.) 
Ref. and Syn.= Mytilites retroflecus Wahl. Upsal.=Avicula retroflewa Hisinger, Leth. Suec. t. 17. f. 12; id. 
“Sil. Syst. t. 5. f.9. Var.a.>%Avicula naviformis Conrad, Journ. Acad. Phil. Vol. VIII. t. 12. f. 11. 
Var. B. >A. erecta id. id. t. 12. f. 5. Var. y. >(A. demissa id. id. t. 13. f.3.=A. ampliata Phil. Geol. 
Surv. Vol. IT. t. 23. f. 1.) = Avicula subretrofewa @Orb. Prod. p. 33. 
Sp. Ch._—Transversely subtrigonal, anterior side short, slightly pointed, tumid, with two faint sinuses in the 
margin, the lower (separating it from the body of the shell) nearly at right angles with the hinge-line, the other 
close to the extremities, inconspicuous; gently convex, posterior wing gradually flattened, the margin sigmoidally 
curved, the curve of the concavity of the dorsal part about equalling the convexity of the ventral portion ; 
surface marked with sharp stric parallel with the margin ; internally a strong bilobed anterior adductor just in 
front of the beak; two slender posterior teeth about two-thirds the length of the hinge-line, and slightly 
diverging from it; a few very small oblique ones beneath the beak. Length of average variety, from beak to 
anal angle, one inch three lines, to end of hinge-line =, width at right angles to hinge-line very variable. 
I find this shell (from examination of many specimens) to vary extremely in the amount of obliquity, no 
two specimens being exactly alike in this respect ; it also varies, in a less degree, in convexity and closeness of 
the striation. Whether the assumed type figured as above by Sowerby is identical with the regularly lined 
shell figured by Hisinger under the same name, is, I think, doubtful. I formerly (Synop. Sil. Foss. Irel. ) 
considered these certainly distinct, from having found in the slates of Kerry a species exactly agreeing with his, 
in the distant regular striation, &c.; a Leintwardine specimen in the Cambridge Collection is, however, exactly 
intermediate, as if in some states of the surface certain of the strize at regular intervals were more prominent 
than the rest. The characters above given embrace, first, the above types in which even the left valve is only 
gently convex, and the beak only slightly prominent, and varying in the closeness and regularity of the strize, 
(var. a.); second, a very convex transversely elongate form, with large prominent beaks, and the dorsal and 
ventral margins subparallel, striation as in the first var. (var. 8.)—to this Conrad has given the name Avicula 
naviformis, and I add two figures of it from Westmoreland specimens (PI. 1. I. f. 9 and 10); third, a variety 
from the Upper Ludlow of Laverock Lane, agreeing with the Avicula erecta of Conrad in the body of the shell, 
being developed nearly at right angles to the hinge-line, but differing therefrom and agreeing with the ordinary 
types in the shorter hinge-line, and the posterior margin having the ordinary equal-ended sigmoidal curve 
(var, y.), and fourth, a broadly rounded, gently convex form, with a very small anterior lobe, and strong con- 
centric strize becoming closer and finer on the posterior ear, the margin of which latter is straighter and meets 
the convex curve of the body abruptly (var. 8.)—this is figured by Conrad and Hall under the name of Avicula 
demissa (for comparison with the good figure of the latter author, I subjoin a figure of a British specimen, 
Pl, 1. I. £. 7, and to shew its passage into the first types I give one intermediate in form and striation, Pl. 1. I. 
f, 8);—as this latter form is important, I have given a separate description of it under its specific name 
P., demissa. . 
Position and Locality—(Var. a.) Aymestry limestone, Leintwardine, Shropshire; in Upper Ludlow 
