328 CARBONIFEROUS LAMELLIBRANCH1ATA. 



greatest anteroposterior length of the valve, and is gently arcuate. The umbones 

 are gibbose, incurved, and twisted forwards, much elevated above the anterior 

 portion of the valve, contiguous and situated in the anterior third of the valve, 

 and excavated in front. The whole valve is regularly curved and very convex, 

 compared to its size, and the dorsal slope broad. 



Interior. — The anterior adductor muscle-scar is pear-shaped, large, shallow 

 and striated from above downwards, placed in the anterior part of the umbonal 

 hollow remote from the margin. The posterior scar has not yet been observed. 

 Details of the hinge and pallial line not known. The cast shows deep ridges and 

 sulci on the internal surface of the valve. 



Exterior. — The shell is ornamented with a few, fifteen to twenty, broad concentric 

 sulci, which are close in front, and all spring from the anterior part of the hinge-line ; 

 they expand as they cross the shell, becoming closer as they approach the posterior 

 part of the hinge-line in which they terminate. These sulci are separated by 

 blunt, imbricating, broad ridges, which become irregular in size and position 

 towards the lower margin, and are very close and small in front. When the test 

 is preserved, the whole of the grooves and ridges are covered by fine concentric 

 lines of growth, one or more of which, here and there, on the sloping lower part of 

 the ridges, are much accentuated. Shell thin. 



Dimensions.— 'Fig. 21, PI. XXXVI. 



Antero-posteriorly . . . .35 mm. 



Dorso-ventrally . . . .25 mm. 



Elevation of valve . . . .16 mm. 



Localities. — England : the Limestone of Thorpe Cloud, Derbyshire. Ireland : 

 the Carboniferous Limestone of St. Dooghlas, co. Dublin, Kildare, and Millicent, 

 Clane, co. Cork. 



Observations. — This species was described and figured by M'Coy in 1844, but 

 subsequently was confounded with another from Lowick (1885), which he 

 erroneously described under the same name. De Koninck drew attention to this 

 fact, and I am in accord with his remarks on the subject, which are as follows : 

 " M. F. M'Coy parait avoir confonder cette espece, qu'il a decrite et figuree en 

 1844, avec une autre qu'il a designee sous le meme nom en 1855. II est vrai que 

 cette derniere s'en rapproche par sa forme et la largeur de ses plis concentriques, 

 mais ceux-ci sont moins nombreux, quoique la taille de la coquille soit a peu pres 

 la meme; elle differe en outre par sa forme generale, qui est beaucouj) plus rec- 

 tangulaire, et par la situation plus anterieure de ses crochets." This shell I now 

 describe as E. MacCoijii, and it may at once be recognised from E. scalaris by its 

 more rectangular form, the anterior position of the umbones, the absence of 

 obliquity, the smaller number of ridges, and the deep quadrate anterior end. 



E. scalaris has externally somewhat the shape of Cardiomorpha, but the 



