68 SYNOPSIS OF THE CHARACTERS OF THE 



centric ridges serve to distinguish it from the others of its genus; the beaks are large, prominent, tumid; an- 

 terior side obtusely pointed ; posterior end rounded; body of the shell very convex. Length six lines, width 

 ten lines. 



ATRACHIA. 



The Atrachia form the second tribe oi t\\e Dlthyra. They are distinguished anatomically from the 

 typical group by the mantle being free, and entirely wanting those long siphons, so characteristic of the Macro- 

 trachia. Most of those shells which possess the power of affixing themselves by a byssus, belong to this tribe, as 

 Byssoarca, Modiola, Pinna, Pecten, &c. 



NucuLA. Lam. 



Gen. Ch. — Equivalve, inequilateral, transverse, hinge linear, with a row of sharp angular teeth on each 

 side the umbones, and a central ligamentary pit. 



NUCULA ATTENUATA. Flem. 

 Nucula attenuata. Flem. Brit. Anim. — Nucula claviformis. Phil. Geol. York. (Not of the Min. Con.) 



Sp. Ch. — Transversely elongated, pear-shaped, gibbous, anterior end rounded, tumid; posterior end com 

 pressed, attenuated, rounded at the tip ; nearly as deep as long, twice and a half as wide as long ; the surface 

 finely striated transversely. Depth three and a half Hnes, width ten lines. 



Nucui-A BiROSTEATA. M'Coy. (PL XL fig. 23). 



Sp. Ch. — Transversely ovate, evenly convex, beaks small, pointed, rather more than a third of the width 

 li-om the anterior end ; posterior end rounded, slightly constricted ; anterior end rounded, narrow, subrostrate ; 

 surface with fine, deep, equidistant, concentric striae, with smooth, flat spaces between them. 



This species at first sight seems closely allied to one of the figures of the Sanguinolaria elliptica, Phil., but 

 is more inequilateral, more finely and regularly striated, the anterior side of the present shell is shorter and 

 narrower than in that, and finally the posterior side, which is broad and obtusely roimded in the S. elliptica, is 

 narrowed or subrostrate, like the anterior end, in this, and the hinge-line which is long and straight in the 

 former, is curved and confounded with the margin of the shell, as in the NucuIcb in the present species ; the 

 posterior mucronation separates it from the N. hrevirostris, Phil. Length five lines, width eight lines. 



NUCTILA BKEVIROSTRIS. Phil. 



Nucula brevirostris. Phil. Geol. York. 



Sp. Ch. — Transversely ovate, anterior side produced, contracted, rounded; surface with fine, concentric 

 striae. 



The produced and contracted anterior side, although a -very common character among the Nucula of the 

 newer rocks, is very uncommon in those of the carboniferous era, and, together with the short ovate form, will 

 distinguish this species. Length three lines, width five lines. 



Nucula caeinata. M'Coy. (PI. XI. fig. 21). 



Sp. Ch. — Rhomboidal, nearly twice as wide as long ; a strong rounded ridge from the beak to the front 

 margin, from which the sides slope gradually ; anterior side pointed, forming with the mesial ridge a nearly 



