Bracuroropa. | LOWER PALAZOZOIC MOLLUSCA. 237 
valve with a very short mesial septum, dividing the small, tumid, reniform pair of muscular impressions, reach- 
ing only one-fifth the length of the shell; from these five or six slender, branched impressions of the pallial 
vessels extend on each side to the margin, a broad border round which is very minutely and closely 
punctured under the lens, the rest of the cast seeming nearly smooth. Width seven lines, proportional 
length varying from ,{, to 7; (the average being about ,{,), depth =. 
This species has some resemblance externally to certain varieties of the L. sericea, but is distinguished by 
the very small number of the large ridges, and their elevation on angular folds ; these ridges, in by far the greater 
number of specimens, are only five, such being the case in all those I originally examined, giving rise to the 
specific name which I have adopted. This character is not so invariable as I supposed, for I have seen one 
specimen in which traces of two other lateral pairs were seen at the margin, and seven may be seen, not very 
uncommonly, but five is by far the most usual; internally it is easily distinguished from ZL. sericea by the 
extremely short muscular impressions. The affinity is perhaps greater with ZL. transversalis, from which it 
differs externally in the same characters above noted, and internally by the much smoother casts, fewer delicate 
impressions of the pallial vessels, and the extreme minuteness of the punctation, differing remarkably from that 
of L. transversalis. Some specimens have small, oblique, concentric, undulating wrinkles on the sides of the 
beak, a few of which sometimes faintly pass across the rostral portion. 
Position and Locality—Bala schists of Cefn Rhyddan, Llandovery, S. Wales; very abundant in the 
Bala schists of Mathyrafal, Meifod, Montgomeryshire; Bala schists of Goleugoed, Llandovery, S. Wales; 
Bala schists of Penlan, Llandovery, S. Wales; Bala schists of Pen y Craig, Llangynyw, Montgomeryshire ; 
Bala schists of Tyn y cabled, Llanfyllin, N. Wales; calcareous schists of Llangynyw Rectory, near Welch- 
pool, Montgomeryshire (one specimen with oblique concentric wrinkles on the ears). Ordinary sized specimens, 
with the oblique wrinkles on the ears, not uncommon in the green, trappean, sandstone of Glenquaple, Ayrshire ; 
in the similar rock at Ardwell, three and a half miles south of Girvan; one very large specimen (six lines long, 
with five principal strize, and one shorter additional one on each side, with minute, undulating, concentric 
wrinkles near the beak), from Colmonel, on the Stinchar; sandy schists of Craig Wen, Mathyrafal, Meifod. 
Explanation of Figures —P\. 1. H. fig. 830. Natural size of transverse variety, with additional lateral 
ridges, from the schists of Mathyrafal; fig. 31, normal type, from the same locality ; fig. 3] a, surface of ditto 
magnified ; 31 4, internal cast, shewing the short muscular impressions and the pallial ridges ; fig. 32, very large 
depressed variety from Colmonel, 
LEPTANA SERICEA (Som.) 
Ref—Sil. Syst. t. 19. f. 1, 2. 
Sp. Ch.—Transversely oblong, substance of the shell thick, very coarsely fibrous; hinge-line slightly 
exceeding the width of the shell; cardinal angles acute, sides gently convex, slightly converging ; front nearly 
straight, wide ; receiving valve much and regularly arched to the deflected margins ; greatest depth at one-third 
from the beak ; entering valve slightly flattened in the rostral portion, rather abruptly arched to the margin; 
greatest depth at two-thirds from the beak; often a few small converging wrinkles on each ear; both valyes 
radiated with extremely fine, hair-like strize, separated by sulci equalling them in size, containing close rows of 
very minute punctures ; at intervals of from one-half to one-fourth (near the margin) of a line apart, certain of 
the strize become about twice the size of the others, including between each pair from five to ten smaller ; about 
thirteen to sixteen striz in the space of one line, in the middle of front margin, at three lines from the 
beak ; cardinal area in each valve low, triangular, that of the receiving valve about a third larger than that 
of the entering valve ; rostral tooth very large, five-lobed, nearly filling the triangular foramen of the receiving 
valve, which is as wide as high, and the apex of which is closed by a small pseudo-deltidium : internal cast 
of receiving valve shewing two very short cardinal teeth, bordering the triangular boss of the foramen, 
diverging at about 135°, from the ends of which proceed two longitudinal, slightly incurved, slightly diverging 
ridges (forming slight furrows on cast), forming the outer boundaries of two long, prominent, obtusely ovate, 
muscular impressions, wide, and only separated near the beak by a very slender sulcus of a mesial septum, 
