Bracuropopa. | LOWER PALAZOZOIC MOLLUSCA. 241 
which extend more than two-thirds the length of the shell, the width of the two together rather exceeding their 
length; a deep, rather wide, mesial hollow intervenes between their prominent boundaries, and in the included 
hollow space of each impression is a very thick pyriform boss, extending nearly its length, and reaching on all 
sides within a short distance of the prominent boundaries, which it exceeds in prominence ; the deflected margin 
striated and punctured. Width seven and a half lines, proportional length of receiving valve %, of entering 
valve 7, depth of receiving valve 74, depth of entering valve >. 
This is a considerably more globose shell than the Leptewna sericea, and distinguished externally by the 
fewer and more distant linear ridges, and the very much finer longitudinal strize between the thread-like ridges, 
and their being besides so faintly impressed as to be, in almost all cases, invisible to the naked eye, or a lens of 
low power, while the strize of Leptena sericea are visible at all times to the unaided vision ; internally the cast of 
receiving valve is distinguished by the remarkable coarseness of a comparatively few punctures, and the 
very short muscular impressions; the cast of the entering valve is equally distinguished by the great pyri- 
form boss, nearly filling the cavity circumscribed for each muscular impression. 
The young of this species, as I have observed in Dudley and Gothland specimens, seems to accord 
with the so-called LZ. lepisma (Dal.) of the Sil. Syst. t. 8. f. 7; and Dalman does not seem to have described 
such a species. 
Position and Locality —Not uncommon in the Coniston (Bala) limestone of Coniston, Lancashire ; common 
in the Bala schists of Llansantfraid, Glyn Ceiriog, Denbighshire; a doubtful specimen in the Bala schists of 
Llwyn y ci, N. W. of Bala, N. Wales; Wenlock limestone of Dudley, Staffordshire; Bala limestone of Bala, 
Merionethshire ; common in the sandstone of May Hill, Gloucestershire. 
Sub-genus. STROPHOMENA (Rajinesque)*. 
Gen. Char.—Shells subsemicircular, depressed ; hinge-line as wide as the shell; flat, or the receiving 
valve slightly convex near the beak, becoming gradually concave or deflected at the margin; entering 
valve flat, or in the deflected species gradually arched downwards with the curve of the opposite valve; 
surface striated, without spines; cardinal area triangular in the large valve, very narrow or obsolete in 
entering valve; deltoidal opening in the cardinal area of the receiving valve nearly closed by a prominent 
pseudo-deltidium, with lines of growth arched towards the beak; interior of receiving valve with a thick 
diverging tooth on each side of the closed foramen; a very short thick mesial ridge, and a narrow semi- 
circular ridge from its end to the cardinal tooth on each side, including a radiated space, the rest of the 
space radiatingly furrowed ; entering valve with the tip of the rostral tooth bilobed, with a strong diverging 
tooth on each side (like that of the other valve), a short mesial ridge, and a small semicircular radiated 
space on each side; the anterior pair of adductors being rarely visible. 
Differs from Orthis in the foramen being almost closed by a pseudo-deltidium (which does not exist in that 
genus), and from the sub-genus Leptwna by the valves not being equally curved in one direction from the hinge. 
LepTana (Strophomena) ANTIQUATA (Sow. Sp.) 
Syn. = Orthis antiquata Sow. Sil. Syst. t. 13. f. 18. 
Sp. Ch.—Truncato-elliptical, or subtrigonal, greatest width along the hinge-line ; cardinal angles slightly 
acute; sides converging to a narrow, obtusely rounded front; entering valve with a gently curved profile, 
slightly flattened near the beak, most curved about the middle of length, greatest convexity along the 
middle, from which the sides slope rapidly to the lateral margins; receiving valve nearly as concave as 
the entering one is convex, but rather convex towards the beak, which is prominent; cardinal area of 
moderate height (about one-sixth of its width), flat, triangular, inclining backwards at about 120°; surface 
of both valves with three or four strong, scaly imbrications of growth, one to two lines apart, interrupting 
* See remarks on Leptcena, page 232. 
[easc. 11. ] Ix 
