202 MOUNTAIN LIMESTONE FOSSILS. 



the C. bipartitum in the crest-like median plate on the boss or 

 central area (axis), but differs in having the axis much smaller, 

 the middle area much larger, the perithecal area smaller, and the 

 fewer lamellse, besides the difference in shape. The C. Keyser- 

 lingiy like the last species, has short secondary lamellae between 

 the longer ones ; it also differs from this in its very slender form, 

 and wants the crest across the axial boss in the cup, but is other- 

 wise nearly allied, although very distinct as a species. 



Common in the carboniferous limestone of Beith, Ayrshire ; 

 rare in the carboniferous limestone of Derbyshire. 



[Col. University of Cambridge.) 



Pteronites persulcatus (M'Coy). 



Sp. Char. Transversely trigonal, right valve gently convex, left 

 valve diagonally tumid, posterior end broad, rounded, flattened 

 anterior end and beaks forming a small, convex, obtusely 

 pointed extremity ; a small space of the anterior extremity 

 smooth, all the rest of the shell covered with small, coarse, 

 rugged, flexuous, irregular ridges, for the most part alternately 

 larger and smaller, and less than their thickness apart, those 

 of the posterior wing nearly straight, radiating, those of the 

 body arching downwards towards the ventral margin. Length 

 of hinge 10 lines, greatest depth at right angles to the hinge 

 5^ lines. 



This species is distinguished from the P. sulcatus (M^Coy) and 

 the P. semisulcatus (M^Coy) (of which latter it has the exact 

 form) by having all the posterior part of the shell striated ; in its 

 ridging it agrees with the Lanistes rugosus (M^Coy, Synop. Carb. 

 Foss. Ireland, t. 10. f. 8), but the above form, and broad beak 

 and anterior end seem to separate the latter. It grows larger 

 than the above measure. 



IN ot uncommon both in the main limestone of Derbyshire and 

 the black limestone resting on it : of large size in the impure 

 limestone of Lowick, Northumberland. 



[Col. University of Cambridge.) 



Streblopteria (M'Coy), n. g. 



Etym. (jTpePXo^, perversus, and irrepov, ala. 



Gen, Char. Ovate or rounded, obliquely extended towards the 

 anterior side; posterior wing broad, undefined, nearly rec- 

 tangular, extending nearly as far as the posterior margin of 

 the shell ; anterior ear small, deeply defined ; surface smooth 

 or radiatingly ridged; one large, faintly marked muscular im- 

 pression a little behind the middle ; one short, narrow tooth 

 slightly diverging from the hinge>line on the posterior sides 



