Prof. F. M'Coy on new Cambro- Silurian Fossils. 403 



and schists of Bala, Merionethshire ; very common in the Cara- 

 doc sandstone of II orderly; very abundant in the schists of Alt 

 y Anker, Meifod, Montgomeryshire ; schists of Tan y bwlch y 

 groes S. of Bala, Merionethshire ; schists of Bryn Melyn near 

 Bala, Merionethshire ; schists of Gelli Grin, Bala, Merionethshire ; 

 schists S.W. of Pwllheli, Caernarvonshire ; schists of Pen y gaer, 

 Cerrig y Druidion, N. Wales; schists of Beavers Grove, Bettws- 

 y-coed, N. Wales; schists of Rhiwargor, &c. 

 (Col. University of Cambridge.) 



Sirophomena sinmlans (M'Coy). 



Sp. Char. Truncato-elliptical ; hinge-line slightly exceeding the 

 width of the shell, forming short, rounded, semicyhndrical ears, 

 from a more or less pronounced indentation of the lateral mar- 

 gins ; front wide, elliptically rounded ; entering valve gently 

 and slightly convex for about nine lines from the beak, and 

 then more rapidly arched towards the receiving valve to a posi- 

 tion sometimes nearly at right angles with the rostral portion ; 

 receiving valve nearly as concave as the entering valve is con- 

 vex externally, except near the beak which is convex ; car- 

 dinal area rather low, triangular, and nearly at right angles to 

 the plane of the lateral margins ; foramen broad, triangular, 

 closed by the pseudodeltidium, except at the apex, where 

 there is af tubular perforation (leaving a little columnar stem- 

 like portion of matrix projecting from the apex of the cast) ; 

 cardinal teeth very small, bifid, diverging at 115°, the dental 

 lamellae, originating from them, converge under the muscular 

 impressions, so that their inner gently incurved edges, touch- 

 ing the shell, diverge at about 65°; muscular impressions 

 undulato-orbicular, about a fifth wider than long, reaching 

 rather less than half the length of the shell ; surface some- 

 times with a few oblique concentric plicae on the ears ; sur- 

 face regularly radiated with very fine slightly irregular striae, 

 about twenty-five to thirty in two lines at six lines from 

 the beak, and at that distance usually every 5th, 7th or 9th 

 of the striae larger than the others, but near the beak they 

 often seem simply alternate in size, the intervening delicate 

 sulci closely punctured. Width 1 inch 10 lines, proportional 

 length of rostral portion about ■$$$, more deflected front about 

 T %% depth at middle of length jjgfc. 



This species in size, form, striation and tubular perforation of 

 the apex of the beak almost exactly resembles the Leptaena alter- 

 nata, Conrad, but differs in having the receiving or foraminated 

 valve concave instead of being the convex one; the striation is 

 also finer and flatter or more uniform, which, with the greater 



26* 



