CARBONIFEROUS FOSSILS OF IRELAND. 71 



NUCULA RECTANGULARIS. M'CoiJ. (PI. XI. fig. 20). 



Sp. Ch. — Subquailrato, very convex, beaks tumid; anterior side very small, rounded; posterior side com- 

 pressed, rectangular, abruptly truncated ; posterior slope undefined, convex ; surface finely and regularly striated 

 concentrically. 



I have seen indications of tlie hinge-teeth in this species, as in Nucula ; at the same time the elevated 

 hinge-margin is very unlike the usual appearance of that genus. It is allied to the N. plicata and N. lucini- 

 formis, Phil., both of which exhibit an angulated posterior side. Every fourth or fifth one of the concentric 

 stri;\i are deeper than the intermediate ones. Length two lines, width nearly three lines. 



Nucula STILLA. M'Coy. (PL XL fig. 18). 



Sp. Ch. — Transversely clavate, gibbous; beak nearly central; posterior side obtusely rounded ; anterior 

 side suddenly contracted, forming a short, mucronate beak, rounded at the extremity ; surface very finely striated 

 concentrically. 



This, although perfectly distinct from all the Palaaozoic species, bears a remarkable similitude to the iV. 

 gutta, Miinster, (A^. mucronata. Sow.): that is, however, a well known Lias species. The beak-like anterior 

 side is very abruptly narrowed from the body of the shell ; the transveise stria3 are very fine. Length three 

 Hnes, width five lines. 



Nucula UNiLATERALis. M'Coy. (PI. XL fig. 17)- 



Sp. Cli. — Transversely ovate, convex; beaks lai'ge, rounded, terminal; anterior side forming a very small 

 subrostral lobe ; ventral margin very convex ; hinge-line straight, two-thirds the width of the shell ; posterior 

 side broad, oval, obliquely subtruncate ; surface smooth, with a few concentric lines of growth. 



This singular shell is rendered curious by having, in fact, no anterior side, the beaks being large, anterior, 

 and terminal, as in Modiola, the anterior side forming a small lobe beneath. It resembles the PuUastra anti- 

 qua, Sow., in form, but wants the elegant striation of that species, and has also a more tumid, prominent beak. 

 Length four lines, width eight lines. 



Arca cancellata. Mart. sp. 



Arcites cancellatus. Mart. Pet. Derb. — Arca cancellata. Sow. Min. Con. 



Sp. Ch. — Nearly twice as wide as long, gibbous; posterior diagonal ridge rounded; surface equally cancel- 

 lated by coarse, radiating, and transverse sulci ; abdominal sinus deep, rounded. 



This species is very rare in the Irish limestone ; the beaks arc large, tumid, approximate ; the abdominal 

 sinus very distinct, and producing a shallow depression down the middle of the shell ; the reticulations on the 

 posterior slope are similar to those on the body of the shell. 



Arca eimbriata. M'Coy. (PI. XII. fig. 8). 



Sp. Ch. — Shell small, very convex, rather more than twice as wide as long; hinge-line equal to the width 

 of the shell ; dorsal and ventral margins parallel ; surface transversely imbricated, longitudinally fimbriated ; 

 posterior margin concave ; posterior side defined by a strong ridge from the beak. 



This is a very beautiful little species ; the lamina of growth are very thick, giving almost a rugged 

 character to the shell ; each one, to the naked eye, appears deUcately fringed ; under the lens tliis is found to be 

 produced by a number of little, short, thick ribs, each having a rounded termination at the ventral edge. The 

 ridge which separates the anterior side is strongly marked, and the whole shell is very convex ; from a slight 

 projection of the hinge-line, the posterior end appears concave. Length two lines, width five lines. The fossil 

 appears to be rare, the only specimen I have as yet seen being the one figured. 



