CARBONIFEROUS FOSSILS OF IRELAND. 105 



internal structure. Among the earliest scientific notices of these shells were those by the celebrated Mr. Martin, 

 whose admirable figures of the more abundant species are, perhaps, the best we have. He formed of them one 

 of his divisions oi Anoinites, under which name most of the fossil Brachiopoda were then described. Mr. Sow- 

 erby subsequently gave this division the name Productus; this term is now usually laid aside in favour of Dal- 

 man's name Leptcena. I have, however, retained the former appellation for the typical genus in the following 

 pages. I have also retained Dalman's name, but use it in a restricted sense, confining it to those flat, scale-like, 

 species analogous to the Leptcena lata, and which abound in rocks of the Silurian range ; while those very ine- 

 quivalve, large, hemispherical species, to which I now'' retain the generic name Producta, are quite unknown in 

 the Silurian rocks, but abound in the carboniferous limestone, where the other form is comparatively rare. I 

 have considered Leptcena, as here restricted, to form a sub-genus of Orthis, rather than as a part of the family 

 Atlnjridce. As the internal structure of the AthyridcB is even less known than that of the Spirifers, I shall, 

 when treating of each genus, lay before the reader what I have been able to discover of it. 



Crania. Retz. 



Gen. Ch. — Subquadrate; upper valve conical; apex nearly central ; lower valve thick, entire, attached by 

 its whole inferior face ; four muscular impressions In each valve. 



Crania vesiculosa. M'Coy. (PL XX. fig. 3). 



Sp. Ch. — Ovato-orbicular, wider than long ; hinge-lino slightly truncated ; apex one-third the length of 

 the shell from the hinge-line, before and on each side of it are placed two obscure tubercles, so situated that, 

 with the beak, they form an equilateral triangle ; surface obscurely undulated concentrically. 



This is the only species of Crania which has come under my notice in the Irish mountain limestone. It 

 is nearly orbicular, or a very little wider than long ; the hinge-margin is obscurely truncated (distinguishing 

 it at once from Orbicula), the surface is rather irregular, slightly imbricated concentrically. This species is 

 allied to the C. prisca. Hen., but that species is longer than wide, while in this the proportion is reversed; they 

 also differ in many minor points. Length eight lines, width ten lines, height six lines. 



Calceola. Lam. 



Gen. Ch. — Triangular, very inequivalve ; cardinal area very broad, flat, triangular, reticulated; dorsal 

 valve semiconic, beak pointed ; ventral valve flat, semicircular ; hinge-line plicated. 



Calceola sandalina. Lam. 



Sp. Ch. — Dorsal valve pyramidal, smooth; beak shghtly, or not at all incurved; cardinal area flat, triangu- 

 lar, marked with decussating striae ; ventral valve flat, semicircular ; hinge-hne sulcated. 



Producta. Sow. (Restricted). 



Gen. Ch. — Shell hemispherical, eared; dorsal valve very convex, ventral valve concave; hinge-line long, 

 equalling or exceeding the width of the shell, compressed ; no cardinal area or foramen. 



The species of this old and extremely natural genus are all very inequivalve, the dorsal valve being very 

 convex, usually with a mesial hollow, and striated longitudinally from the beaks. The ventral valve is con- 

 cave, generally with a mesial elevation, corresponding to the depression in the dorsal valve. The position of the 

 muscular impressions differs essentially from that of the Spirifers; they are also considerably larger than those 

 of that genus. The two pair of principal adductors arise one pair on each side of the beak of the dorsal valve, 

 forming a large square cicatrix ; these form elevations in the cast. The principal pair of adductors of the ventral 



^ The above was written three years ago. 

 2D 



