Bracutopopa.] LOWER PALAOZOIC MOLLUSCA. 233 
becoming by age deflected usually towards the receiving valve: e.g. L. euglypha (Dal.) and Strophomena 
rugosa (Raf.) 3rd, Leptagonia (M*°Coy), with both valves abruptly bent at right angles towards the entering 
valve, and the rostral portion concentrically wrinkled. 4th, Chonetes (Fischer), only differing from Leptena 
(as restricted) by having a row of spines on the hinge-line, thus approaching Productus. All the above 
types, including all Dalman’s species, have a foramen between the beaks, contrary to his definition. 
Sub-genus. LEPTAINA restricted (see remarks on Genus). 
LEPTANA ALTERNATA (Conrad.) 
Ref. and Syn.—Geol. Rep. New York, 1838, and Hall, Pal. New York, t. 31 a. f. 1. and t, 79. f, 2. 
Sp. Ch.—Longitudinally semielliptical or transversely subquadrate; receiving valve gently convex, along 
the middle, with a very short mesial sulcus close to the beak, gently deflected at the side margins when 
old; entering valve flat, or slightly concave; cardinal area very narrow, inclined back at about 120°: surface 
of both valves radiated with very fine linear thread-like strize, separated by equally wide, flat, coarsely 
punctured spaces, when partially decorticated ; striee thicker and closer together, when the surface is pre- 
served ; strie of two, more or less distinct sizes, usually three of the finer, between each pair of the larger ; 
the middle subsequently increasing in size, and a new pair of small strie being intercalated on each side; 
about eighteen strize in the space of two lines at six lines from the beak: interior of the valves marked 
with radiating external strize, and with very numerous, obtuse granules, nearly corresponding with the ex- 
ternal punctures. Length one inch, width varying from a little more to a little less than the length, 
proportional depth about =, 
It may, I think, be doubted, whether this will not ultimately prove a mere variety of Strophomena 
? compressa, the internal characters both of the entering and receiving valve as figured by Hall, and of the 
muscular impressions and ridges, and cardinal teeth, agreeing precisely with those of the Strophomena ? 
compressa, as I have observed; the only differences being that the larger strie are more distinct, and the 
large valve more convex in the alternata than in the compressa; and in the latter I have not seen the 
small foramen in the apex of the beak, 
Position and Locality——Common in the Bala limestone of Balmae Shore, Kirkcudbright; ?a flattened 
variety in the limestone and sandstone of Dalquorhan, near Girvan, Ayrshire. 
LEPTANA CORRUGATA (Porth. Sp.) 
Ref. and Syn. = Orthis 1d, Portlock, Geol. Rep. t. 32. f. 17, 
Sp. Ch.—Semiorbicular; receiving valve gently convex; hinge-line slightly exceeding the width of the 
shell; surface radiated with strong, narrow, simple, prominent, slightly divaricating ridges, running straight 
along the hinge-line, increasing irregularly towards the margin by intercalation, about five in two lines at 
two lines from the beak, or six in the same space at four lines; intervening spaces flat with from three 
to seven fine, longitudinal strize between each pair of ridges, and crossed by large irregular wrinkles, about 
the width of the spaces apart, which do not cross the ridges nor form regular concentric lines; all the strix 
crossed by extremely minute, close, regular lines of growth. Width ten lines, proportional length & to 2. 
I have not had an opportunity of examining either the entering valve, the interior, or the cardinal area 
of this extremely beautiful and distinct species, which however I place in Leptwna on the authority of 
M. Barrande (see his Memoir, “ Ueber die Brach. der Sil. Schich. von Bohmen,” in Haidinger’s Natur- 
wissenschaftliche Abhandlungen, Band 2), where however he figures the large ribs both of this species and 
the adjoining Z. sericea much too far apart: he represents the entering valve nearly as concave as the 
receiving is convex, and the cardinal area very low, 
Position and Locality,—Rare in the schists of Goldengrove, Llandeilo, 
[Fasc 11.] Hu 
