Bracuiopopa. | LOWER PALAZOZOIC MOLLUSCA. 239 
Var. a. RHOMBICA (J/°Coy). 
In this variety the shell is more gibbous in the middle than in the true types, and less transverse, the 
proportional length varying from 5 to 2, as compared with the width, and the strong ridges fewer and 
often nearly a line apart, sometimes elevated on obtuse folds near the margin, in those respects approaching 
the L. transversalis, from which however it differs, in having the intervening strixe of the same size and 
strength as the ordinary types, wanting the remarkably large punctures on the interior, &c.; the muscular 
impressions also agree rather with the Z. sericea than with the ZL. transversalis. 
Position and Locality (for rhombica).—Not uncommon in the Bala schists of Mathyrafal, S. of Meifod, 
Montgomeryshire; Bala limestone of Horton in Ribblesdale, Yorkshire; Bala schists of Tyn y cabled, 
Llanfyllin, N. Wales, and Bala schists of Pen-y-craig, Llangynyw, Montgomeryshire; Bala schists of Cefn 
Rhyddan, Llandovery, S. Wales ; fine sandy Bala schists of Alt yr Anker, Meifod, Montgomeryshire; schists 
of Llyn Alwen, Denbighshire; Bala schists of Gelli Grin, Bala, Merionethshire. 
Var. 8. SPINANGULA (Phil.) 
30 
The var. spinangula of Phillips (Mem. Geol. Surv.) has the length sometimes only % in proportion to 
the width. 
LEPTHNA TENUICINCTA (M°Coy). Pl. 1. H. fig. 40. 
Ref—Ild. M°Coy, Ann. Nat. Hist. 2nd Series, Vol. VIII. p. 401. 
Sp. Ch.—Rotundato-quadrate ; hinge-line as long as the shell is wide; cardinal angles forming nearly 
rectangular, small, flattened ears; sides subparallel, obtusely rounded, front wide; receiving valve subhemi- 
spherical, very gibbous, greatest depth a little behind the middle of the shell; beak large, prominent, obtuse, 
incurved to the level of the lateral margins; cardinal area nearly in the plane of the lateral margins, very broad, 
flat, triangular, height rather more than one-fourth the width; triangular foramen narrow, closed by a convex 
pseudo-deltidium ; entering valve very concaye, with a mesial septum extending nearly to the margins; surface 
of both valves marked with very regular, minute, equal, rounded, concentric wrinkles, having a slight retral wave 
in the middle of the front, about twelve in the space of one line at the middle of the shell. Width about five 
lines, proportional length of receiving valve ;\, of entering valve 4%, depth =. 
This beautiful species seems almost identical in size, shape, and marking, with the Producta tenwicineta 
(M°Coy), and Leptena enigma (Vern.), but has a very wide distinctly-marked cardinal area. 
Position and Locality.—Olive Bala schists of Cefn Grugos, W. of Llanfyllin, Montgomeryshire. 
Explanation of Figures.—P\. 1. H. fig. 40. Natural size, shewing the entering valve, with the cardinal 
area and pseudo-deltidium of the receiving valve; fig. 40a, profile of receiving valve; fig. 40 4, longitudinal 
section ; fig. 40 c, striation of surface magnified. 
LEPTAINA TENUISSIMESTRIATA (M‘Coy). Pl. 1. H. fig. 44. 
Ref. and Syn.? = Orthis lata Sow. Sil. Syst. t. 22. f. 10?= Orthis tenuissimestriata M*Coy, Sil. Foss. 
Irel. t. 3. f. 20. 
Sp. Ch.—Transversely oblong, or slighty fusiform, about twice as wide as long, moderately convex, 
regularly arched, profile nearly semicircular, greatest depth about the middle of the length; entering valve 
flattened, about one-third the depth of the receiving one; hinge-line equalling the width of the shell; cardinal 
angles very slightly acute in depressed specimens, but sometimes forming acute convoluted ears; beaks very 
small: surface of both valves radiated with very close, fine, slightly unequal strize, often subalternate near the 
margin, every third or fifth being often a little larger than the others, increasing principally by intercalation of 
new strize, size nearly uniform from the beak to the margin ; intervening sulci much narrower than the ridges, and 
