CARBONIFEROUS FOSSILS OF IRELAND. 79 



The hinge-line is very short and oblique, the beak is placed nearly in the centre of it. From the beak to the 

 posterior extremity one inch six lines, length six lines. Specimens frequently occur nearly tliree inclics in 

 tlieir longest diameter ; tlie figure is from a shorter specimen than usual. 



POSIDONIA SIMILIS. M'Coy. (PI. XII. fig. 2). 



Sp. Ch. — Obliquely ovate, with numerous, irregular, small, rounded, concentric ridges; beak nearest the 

 anterior e.xtremity. Length two inches, breadth about one-fourth less. 



This species is well distinguished from tlie P.Becheri, by its small, rounded, cord-like undulations, instead 

 of the regular, sharp, concentric ribs which mark that species. 



POSIDONIA TUBERCULATA. SoW. 

 Posidonia tuberculata. Sow. Geol. Trans — Posidonia tuberculata. P/iil. Pal. Foss. 



Sp. Ch. — Obovate, oblique, compressed; numerous concentric, angular ridges; decussated and tubercu- 

 lated by four or five longitudinal, radiating ribs from the beak. 



This species is scarcely distinguishable from the P. Becheri; the principal character Is the existence of a 

 few longitudinal ribs in the present shell which decussate the concentric ridges, forming tubercles at each of 

 the crossings ; but these ribs are so variable in number and position, and are often so obscure, that it is doubtful 

 whether they form a good specific character ; in some specimens, however, they are very remarkable ; the con- 

 centric ribs are usually fewer and farther apart than in the P. Becheri, but this is a still more variable charac- 

 ter. Length two inches two lines, width two inches one line. 



Posidonia ? (PI. XXL fig. 3). 



This little species was found in company with the P. similis, M'Ooy. It resembles the Posidonia Becheri 

 in the sharpness of its concentric ribs, but differs from it in having them larger and wider apart, and in having 

 the beak in the centre of the liinge-line. 



Meleagrina. Lam. 



Margarita. Leach. 



Gen. Ch. — Shell rounded, auricles small; umbones sublateral; hinge-line straight, but not lengthened or 

 oblique ; teeth small, tuberculated, obsolete. 



I have found it convenient to extend this genus to all those AviculcE whose hinge-line is rectangular, and 

 the general form rounded, thus leaving the true AviculcB a very well defined group. 



Meleagrina alternata. M'Coy. (PI. XIIL fig. 17). 



Sp. Ch. — Truncato-orbicular, convex ; ears large, nearly equal, marked with very strong, concentric striae ; 

 body of the shell radiated with about eighteen strong, slender, distant ridges, alternately longer and shorter, the 

 intermediate spaces being marked with rather distant, regular, concentric wrinkles. 



In this minute but beautiful shell, the ears are nearly equal in size, one being rounded, the other pointed ; 

 they are without radiating ridges, but are very strongly marked with rounded, coarse striffi, parallel with the 

 outer margin. Length one line, width one line. 



Meleagrina echinata. M'Coy. (PI. XIII. fig. 18). 



Sp. CIi. — Orbicular, compressed, thin; hinge-line rather less than the width of the shell; anterior ear 

 equal to the posterior ear in length, narrow, rounded, deeply divided from the body of the shell; posterior ear 

 undefined, rectangular ; surface marked by rather distant, narrow, radiating ridges, strongest towards the mar- 



