122 CARBONIFEROUS LAMELLTBRANCHIATA. 



now known that at least three species have well-marked internal ribs, and there- 

 fore they are referred to Amusium. It is unnecessary to discuss the question of the 

 relation of this genus to Syncyclonema, to which it is so closely allied in shape and 

 structure, as this has been fully done in my observations on that genus, p. 11 G. 



Amusium concentricum, sp. nov. Plate XXI, figs. 7 — 11. 



Entolium Sowerbyi, de Konineh, 1885. Ann. Mus. Boy. d'Hist. Nat. Belg., torn. 



xi, p. 241, pi. xxxiii, figs. 5, 6. 

 Aviculopecten ? protetts, de Koninck, 1885. Ibid., p. 239, pi. xxxiii, figs. 3, 4; 



pi. xxxix, fig. 12 ; and pi. xl, figs. 10—13, 16—20. 



Specific Characters. — Shell small, ovate from above downwards. The central 

 part of the valve narrow, triangular, and only moderately convex, separated by 

 broad, oblique grooves from an anterior and posterior broad flange-like process. 

 The margin regularly rounded in the greater part of its extent, when it becomes 

 deflected both in front and behind towards the middle line, forming an obtuse 

 angle. The hinge-line short and straight, or angular, the ears very small, the 

 posterior the longer and larger. The umbones small, pointed, and central. 



Interior. — The internal surface is ribbed. 



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

 growth, through which are seen obscure, almost obsolete, broad, widely separated, 

 radiating ribs. 



Localities. — England : the Carboniferous Limestone of Thorpe Cloud, Park 

 Hill, and Castleton, Derbyshire; Hill Bolton, Yorkshire; above the Main Lime- 

 stone, Nine Standards Rigg, Westmoreland. Ireland: the Carboniferous Limestone 

 of Little Island, co. Cork; Lisbellaw, co. Fermanagh; Croag, co. Limerick; St. 

 Doulaghs, co. Dublin. 



Dimensions. — PI. XXI, fig. 7, measures — 



Antero-posteriorly . . . .28 mm. 



Dorso-ventrally . . . .34 mm. 



Observations. — This little shell is not at all rare in the upper beds of the 

 Carboniferous Limestone of Derbyshire. It has probably always been regarded 

 as the young of S. Sowerbyi; indeed, I have no doubt that de Koninck figured 

 and described a specimen under this name. A reference to his figure [loc. cit.) 

 will show that his shell had radiating ribs and sulci, a character which at once 

 separates A. concentricum from that shell. A. concentricum is much narrower than 

 S. Sowerbyi, and lias the concentric markings well developed, the latter shell being 

 almost smooth and also much larger and suborbicular. 



