CARBONIFEROUS FOSSILS OF IRELAND. 83 



AviCULA ANGUSTA. M'Coi). (PI. XIII. fig. 20). 



Sp. C/;.— Obliquely elongate, depressed, nearly three times as wide as long; posterior slope and ventral 

 margin straight and parallel ; posterior slope very sliglitly marked ; a slight sinus in the ventral margin, op- 

 posite the beak ; posterior end obtusely rounded ; posterior wing small, falciform, smooth ; body of the shell 

 marked with very delicate, concentric, imbricating striae. 



This resembles the GervilUa inconspicua, Phil, but is narrower, and the wing is mucli smaller, and diffe- 

 rent in form. Length four lines, widtli ten lines. 



AviCULA BICOSTATA. M'Coi/. (PI. XIIL fig. 26). 



Sp. CIi. — Transversely oblong, twice as wide as long, very gibbous; beaks large, tumid; anterior side 

 short, rounded ; posterior side lengthened, rounded ; hinge-margin pointed, slightly shorter than the shell ; a 

 deep sinus e.xtends from the beak to the opposite ventral margin, bounded on each side by a broad, distinct 

 flattened rib ; entire surface sharply and regularly striated concentrically. 



This very beautiful shell is distinguished from all the allied fossils, by the large ribs bounding the abdo- 

 minal sinus, and the elegant striation of the surface. Length two lines, width four lines. 



AviCULA FLABELLULA. M'Col/. (PL XIII. fig. 27)- 



Sp. Ch. — Obliquely ovate, convex; ears small, without radiating strise ; surface finely striated from the 

 beaks ; ridges flat, each divided by a fine line in the centre, and separated by a narrow, concave sulcus. 



This species is remarkable for the very great width of the front, which is fully double that of the hinge- 

 line ; the beaks are small and prominent ; the ears are small, nearly smooth, or with a slight, irregular plication, 

 parallel to the margin ; the surface is very regularly radiated from the beak, with fine, smooth, flattened, 

 elevated stria, most of which are divided by a fine line about the middle. Length seven and a half lines, width 

 nine lines. 



AviCULA GIBBOSA. M'Coy. (PI. XIII. fig. 25). 



Sp. Ch Obhquely oval, short, very gibbous; beaks large, prominent, incurved; posterior wing small, 



acute, falcate ; anterior side moderate, rounded ; posterior side semielliptical, abdominal sinus deep ; surface 

 with a few, irregular, obtuse lines of growth. 



This Httle species is chiefly remarkable for the great convexity of its valves; the posterior slope is 

 gradually rounded ; it is less elongated transversely than most of the genus ; the posterior, wing-like termina- 

 tion of the hinge-line has a few coarse striae parallel with the outer margin ; the smoothness of the surface is 

 interrupted by irregular, ^^romincnt lines of growth. Length three lines, width five Imes. 



AviCULA iNFORMis. M'Coy. (PL XIII. fig. 21). 



Sp. Ch. — Transversely trigonal, beaks obtuse, nearly terminal; anterior end very small; posterior end ob- 

 tusely rounded ; hinge-line as long as the shell, forming a rectangular, compressed wing ; body of the shell very 

 ffibbous ; surface smooth, with two or three large, obtusely rounded, concentric wrinkles, most distinct over the 

 posterior slope. 



This little species is not uncommon; its few obtuse wrinkles and general bluntness of form approximate it 

 to the A.quadrata, M'Coy, but it is much more transverse, and smaller. Length one line, width two lines. A 

 very large specimen measures two lines in length, and four m width. 



