MOUNTAIN LIMESTONE FOSSILS. 207 



beak, about the'middle of the shell); periostraca sharply marked 

 with close interrupted stride and a few minute scattered points, 

 very rarely falling into close regular radiating lines ; usual 

 width 9 lines, length 1 inch 3 lines, greatest depth (a little 

 behind the middle) 7 lines (occasionally 2 inches long) . 



The irregular interruption and undulation of the concentric 

 wrinkles in front of the middle of the sides is often very striking 

 and beautiful and is always recognizable. It is veiy variable in 

 the thickness and regularity of the ridges ; it most nearly ap- 

 proaches the S. regularis (King sp.), from which it differs in the 

 undulatory interruption of the ridges at the place mentioned, 

 and in being shorter, and the greatest gibbosity of the shell 

 being along the anterior boundary of the posterior slope — it 

 being much nearer the anterior end, and the posterior portion 

 being compressed in that species, in which also the greatest gib- 

 bosity is nearer to the dorsal margin, giving a much less tumid 

 character to the lower part of the valves. From the >S^. sulcatus 

 it differs in the wrinkles not uniting into few large wrinkles in 

 passing to the posterior slope, &c. 



Rare in the carboniferous limestone of the Isle of Man ; not 

 uncommon at Lowick, Northumberland. 



{Col. University of Cambridge.) 



Leptodomus costellatus (M'Coy). 



Sp. Char. Oblong, very tumid, width three-fifths of the length, 

 depth of both valves about equal to the width ; anterior and 

 posterior lunettes large, defined ; beaks large, tumid, incurved, 

 terminal, anterior side obtuse, subtruncate, slightly oblique be- 

 neath them ; a small sinus in the ventral margin close to the 

 anterior end, from which a narrow concavity extends nearly 

 to the beaks close to the anterior edge; hinge-line nearly as 

 long as the shell, with a shght upward curvature ; posterior end 

 wide, slightly oblique, subtruncate, rounded ; ventral margin 

 strongly convex behind the sinus; posterior slope abruptly 

 compressed, smooth, or with a few lines of growth parallel 

 with the margin, divided nearly in the middle by a small fur- 

 row from behind the beaks; sides marked with numerous 

 small, regular, close, narrow, roiinded ribs parallel with the 

 margin (about four in the space of 2 lines) ; these abruptly 

 disappear on reaching the edge of the posterior slope, and 

 unite on the anterior edge in front of the sinus in parcels of 

 two or three to form a row of short thick wrinkles on that 

 part. Length 1 inch 4 lines, width 10 lines, depth 10 lines, 

 width of posterior lunette 1^ line. 



I long imagined this to be the Hiatella sulcata of Fleming, 



