LAMELLIBRANCHIATA. } LOWER PALAZOZOIC MOLLUSOCA. 275 
very deep, wide, sharply defined, cordate, anterior lunette ; anterior side very small, scarcely extending as far as 
the beak; anterior half of the ventral margin slightly convex, a faint sinus in the posterior half; sides and 
posterior slope steeply inclined, slightly convex towards the anterior end; hinge-line about half the length of 
the shell, slightly elevated (the margins simple and erect, with a moderately slender lateral tooth, extending a 
little below it); posterior end obliquely truncated; cardinal angle very obtuse (about 150°), respiratory angle 
pointed (about 60°), length eleven lines ; greatest width, in proportion to length, at end of hinge-line “%, greatest 
depth of one valve (about the middle) ;. (Larger specimen), length one inch five lines, proportional length of 
hinge-line ;;;, width from middle of hinge-line at right angles to ventral margin =, width of posterior end =, 
depth of one valve =. 
Position and Locality.—Tilestone of Horeb Chapel, Llandovery, S. Wales; Upper Ludlow, Burton and 
Brockton, near Wenlock. 
ORTHONOTUS NAsUTUS (Conrad Sp.) Pl. 1. I. fig. 23. 
Syn. and Ref. = Cypricardites nasutus (Conrad) fid. Hall. = Modiolopsis nasuta id. Pal. N. Y. t. 18. f. 2. 
Sp. Ch.—Elongate, narrow, subcylindrical or elliptical; gently convex ; dorsal and ventral margins sub- 
parallel; beaks moderate, nearly one-fourth of the length from the anterior end, which is greatly narrowed by 
the very large anterior lunette, rounded, and slightly recurved at the tip; sides gently convex ; posterior slope 
steep but obscurely defined; posterior end obliquely subtruncate, elliptically rounded; surface with faint 
concentric lines of growth. Length one inch three and half lines, width >, length of anterior end %, depth =. 
This agrees minutely in every respect with the American species ; which according to Hall is common in 
the arenaceous strata near the top of the Hudson Riyer group. 
Position and Locality Rare in the Caradoc sandstone of Horderly West, Shropshire. 
Explanation of Figure —P1. 1. 1. fig. 25. Natural size, right valve. 
ORTHONOTUS SEMISULCATUS (Sow. Sp.) Pl. 1. K. fig. 25. 
Syn. = Modiola? semisulcata Sow. Sil. Syst. t. 8. f. 6. 
Sp. Ch.—Oblong, valves gently convex, most so from the beak towards the respiratory angle, behind which 
the posterior slope inclines rapidly to the hinge-line; side slightly flattened, or concave along the middle; 
beaks prominent, rather less than one-sixth of the length from the anterior end, which is compressed, sub- 
angularly narrowed in the middle, and abruptly sloping both from the beaks and ventral margin; anterior 
lunette deep and large, posterior end slightly narrowed, obliquely subtruncate, obtusely rounded; dorsal margin 
straight, not elevated, a thick cardinal ridge just behind it; ventral margin slightly concave in the middle ; 
surface with fine concentric strize, a few irregularities of growth, nearly smooth, anterior end strongly wrinkled 
longitudinally. Length one inch four lines, proportional width ;;;, length of anterior end 4, depth 2, hinge- 
line from beak to anal angle =. 
The form and wrinkling of the anterior side approximate this species to the Grammysia triangulata, from 
which it differs in its more angulated anterior side, and in its want of the oblique divisional sulci from the beak. 
The dorsal edges are erect, with a thick internal ridge below them as in Hdmondia, to which it also approxi- 
mates by the large anterior lunette, differing from that genus only in the slight sinus (for byssus?) in the 
margin and corresponding hollow in the sides. 
Position and Locality —Not uncommon in the green quartzite (Upper Ludlow), Kirkby Moor, Kendal, 
Westmoreland ; rare in the Bala sandstone of Mulock Quarry, Dalquorhan, near Girvan, Ayrshire. 
Explanation of Figure.—Pl. 1. K. fig. 25. Left valve, natural size ; from near Kendal. 
3rd Family. CASLONOTIDA (M'Coy). 
Shell elongate, gradually widening to the posterior end, which is slightly gaping; usually a shallow byssal 
furrow from the beaks to the anterior third of the ventral margin; hinge-line nearly as long as the shell ; 
NN2 
