CARBONIFEROUS FOSSILS OF IRELAND. 121 



LEPTiENA SERRATA. M'Coy. (PI. XVIII. fig. JO). 



Sp. Ch. — Semicircular, convex ; surface covered with numerous, ratlicr coarse, branched ridges; hinge-line 

 equal to twice the length, straight, furnislicd with twenty small, hooked spines. 



This species has been confounded with the L. lata of the Silurian rocks. It has, however, twenty short, 

 hooked spines on the hinge-line, while that has but eight, or ten at most, long and slender ones. The thick, 

 dichotomous striae are likewise so characteristic, as to enable the geologist to distinguish even small fragments 

 of the two species. Breadth ten lines, length five lines. One of the rarest fossils we have. 



LePTJENA SORDtDA. SoW. 

 Leptaena sordida. Sow. Geol. Trans — Ortliis sordida. Phil. Pal. Fos. 



Sp. Ch. — Outline from subquadrate to nearly semicircular; slightly convex; finely striated longitudinally; 

 striaj granulated near the margin. 



This pretty species is nearly allied to the O. Hardrensis, Phil., but is distinguished by its greater length 

 in proportion to the width, and the strise being granulated near the margin ; the internal structure is also very 

 different. Length four lines, width six and a half lines. 



Lept^na tuberculata. M'Coy. (PI. XX. fig. 5). 



Sp. Ch. — Semicircular, convex ; length two-thirds the width ; surface with about forty-eight thick, 

 rounded, dichotomous, smooth ribs, each bearing towards the margin a row of from five to eight round tubercles. 



This curious little species is nearly semicircular, the hinge-line being much shorter than the width of the 

 shell ; moderately convex ; the radiating ribs are thick, smooth, and distinctly separated, branching as they 

 approach the margin, where they bear a row of six or eight little round tubercles. Length five lines, width 

 eight lines. 



Lept^na volva. M'Coy. (PI. XVIII. fig. 14.) 



Sp. Ch. — Semicircular, gibbous; ears involute, separated from the body of the shell by a shallow depres- 

 sion ; hinge-line twice the length of the shell, furnished with twenty-four slender, hooked spines ; surface 

 covered with very fine, flexuous striee. 



This pretty species resembles the L. convoluta in general form, but is distinguished by the exceedingly 

 fine, flexuous striae ; the two valves are almost in contact, the ventral valve being nearly as concave as the dor- 

 sal is convex ; the cardinal area is very narrow, concave, and with parallel sides. Length eight lines, width one 

 inch four lines. 



Orthis. Dal man. 



Gen. Ch. — Shell semicircular; dorsal valve convex in the middle; ventral valve nearly flat, slightly con- 

 vex or concave ; cardinal area triangular, common to both valves, that of the dorsal valve reticulated by 

 muscular striae, a large triangular foramen in the centre, nearly closed by arched scales ; no spiral appendages ; 

 dental lamellae semicircular, curving towards each other at their extremities. 



Orthis arachnoidea. Phil. 



Spirifer arachnoidea. Phil. Geol. York. — Orthis arachnoidea. Phil. Pal. Fos. 



Sp. Ch. — Semielliptical, flat; hinge-line equal to the width of the shell; length exceeding the width ; 

 surface with numerous sharp, fine, close, divaricating striae, increasing very much in number towards the 

 margin, a few equidistant sulci deeper than the rest. 



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