300 CARBONIFEROUS LAMELLIRRANCHIATA. 



portions, of which the anterior is the larger, and this part of the valve is convexly 

 swollen. Posterior to the ridge the valve is so rapidly compressed as to be 

 concave along the dorsal slope. The limule is large and cordate; the escutcheon 

 large, deep, and elongate, deeper and wider behind than anteriorly. 



Interior. — The anterior adductor muscle-scar is small and deep, situated 

 immediately within the anterior superior angle, and separated from the rest of the 

 shell by a ridge. The posterior adductor scar is small and shallow, situated at 

 the posterior superior angle some little distance from the margin. Pallial line 

 entire, very finely marked, and remote from the margin. The hinge consists in 

 the right valve of a single cardinal tooth with a shallow pit on each side, and an 

 elongate groove between two ridges posteriorly to receive the tooth of the 

 opposite valve. The left valve possesses a central pit for the cardinal tooth of 

 the right valve, with a rudimentary tooth on each side, and a single, long, lamellar 



posterior tooth. Formula T ~ ' , „ , '- , • The inferior and posterior edges of the 



valve are finely serrate on the internal aspect, as in Astarte. 



Exterior. — The surface is ornamented with very concentric, fine lines of 

 growth, but here and there one of them becomes highly accentuated, especially 

 near the lower border. The umbonal region and the upper part of the valve are 

 almost smooth. 



Dimensions. — Fig. 18, PI. XXXIX, a specimen from Orchard, in the collection 

 of Mr. J. Neilson, measures — 



Antero-posteriorly . . . .11 mm. 



Dorso-ventrally . . . .7 mm. 



From side to side . . . .8 mm. 



Localities. — Scotland : the Upper Limestone series of Orchard ; Linn Spout, 

 Dairy; Robroyston; Gare and Westerhouse, Carluke; Williamswood, Cathcart ; 

 Auchentibber, Kilwinning; Lower Limestone series, Law, Dairy. 



Observations. — This species was described as occurring in Scottish Carboni- 

 ferous beds by Armstrong. It can be distinguished at once from all the smaller 

 species of the genus by the strong, acute, oblique keel, which gives the shell a 

 very characteristic contour when viewed from above. De Koninck has figured 

 i liis species apparently from the Limestone of Vise, but from his language there 

 appears to have been some doubt as to the locality. He says, " Cette espece 

 parait avoir ete trouvde dans le Calcaire Carbonifere de Vise." 



I have been fortunate enough to isolate the hinge in a left valve in the 

 collection of Mr. J. Smith, of Kilwinning, fig. 15, PI. XXXIX. The other 

 details have been obtained from mere fragments; in hinge structure and internal 

 anatomy C. acuticarinata agrees entirely with those of C. crebricostata, of which 

 some fine and perfect valves have been obtained. 



