CARBONIFEROUS FOSSILS OF IRELAND. 167 



Cythere inflata. B'PCoy. (PI. XXIII. fig. 17). 



Sp. Ch. — Shell gibbous; anterior and posterior ends nearly alike, anterior a little larger; diameter from 

 the most convex part of one valve to that of the other, five-sixths of the depth, measured from the centre of the 

 dorsal to the centre of the abdominal margin ; depth two-thirds of the length ; abdominal margin of the left 

 valve overlaps. Length from one to two lines. 



This is the largest, as it is Hkewise the most abundant species of Cythere found in the mountain limestone 

 of Ireland. Its most ordinary size is about a line and a half long, and one line from the dorsal to the abdomi- 

 nal margin ; the back is much and regularly curved, greatly inflated, and marked along the centre by a groove 

 or sulcus, which points out the situation of the hinge ; towards the abdominal margin it suddenly becomes com- 

 pressed, the abdominal margin itself is straight, without any sinus, and presenting a very small, obtuse, radicle- 

 like projection ; the abdominal portion of the left valve overlaps the other very distinctly. 



This species abounds most in dark, fa3tid limestones, but it is rai-e in the light-coloured limestones, where 

 the following species supplies its place. 



Cytheke iNORNATA. M'Coy. (PI. XXIII. fig. 18). 



Sp. Ch. — Compressed; diameter one-half the depth; sides slightly convex; shell plain. 



This is a very small species ; rarely exceeding half a line in length, it bears a great resemblance to the 

 young of the C. corwMte, M'Coy, in general form, but wants the characteristic spines of that shell, and the sides, 

 instead of being concave, are convex. It is also much more compressed than the C. iitflata, M'Coy, which it re- 

 sembles in the want of spines. In that shell the depth is less than the diameter, while it is double the diameter 

 in tills species. Very common in several localities. 



Cythere oblonga. il/'C'oj/. (PI. XXIII. fig. 22). 



Sp^ Ch. — Oblong, very gibbous; length rather less than twice the depth; dorsal and ventral margins 

 slightly curved, parallel ; ends almost equal, very obtuse, rounded ; surface smooth, without ornament. 



This is rather a large species, having, to the naked eye, an oblong square form ; the valves have their 

 greatest convexity in the middle, while in the C. Hibberti, M'Coy, and C. injiata, M'Coy, which it most re- 

 sembles, the greatest convexity is along the dorsal margin ; from those species it is likewise distinguished by 

 its greater length and the equal curvature or paralleHsm of the dorsal and ventral margins. Length about one 

 line. Common. 



Cythere orbicularis. M'Coy. (PI. XXIIL fig. 19). 



Sp. Ch. — Nearly orbicular, depressed, length very slightly exceeding the depth ; valves flattened, gibbous 

 in the middle, smooth. 



This is comparatively a large species, nearly equalling the C. injiata and C. Scotohurdigalensis in size, from 

 boih of which it is easily known by its proportionate shortness, the flatness of the valves, and their central gib- 

 bosity ; from the C. gihberula, which it resembles in the central gibbosity of the valves, it is distinguished, as 

 in the other cases, by the remarkable shortness in proportion to the depth. Length one Hne. 



Cythere pusilla. M'Coy. (PI. XXIIL fig. 20). 



Sp. Ch. — Oval, length but slightly exceeding the depth, very gibbous; ends rounded; ventral margin 

 short; sm-face smooth. 



This is the smallest species of Cythere which I have observed in the Palffiozoic rocks ; it has no very tan- 

 gible characters, yet from its great abundance it becomes important ; it is more nearly spherical than any of the 



