166 SYNOPSIS OF THE CHARACTERS OF THE 



for a small space beneath. This species bears a considerable resemblance to the Cij-pris spinigera, Sow., but 

 that shell is an inhabitant of the Wealden rocks, and truly belongs to the fresh water genus, Cypris, while this 

 is a marine shell, and consequently not of that genus. A comparison of the figures of the two species will 

 exhibit numerous differences. Total length one line ; the diameter is two-thirds the depth ; depth a little more 

 than half the length. Not common. 



CyTHERE ELONGATA. M'Coy. (PL XXIII. fig. 13). 



Sp. Cli. — Three times as long as deep; dorsal and ventral margins nearly parallel, very slightly curved; 

 ends equal, rounded ; sides convex, smooth, one small tubercle near the anterior end. 



The great length in proportion to the depth, is so remarkable in this species, that although very small, it 

 is easily distinguished by the naked eye. It is very common in the shales of certain localities ; but, from the 

 want of a specific name, has not been heretofore noticed. Length half a line. 



Cytheee excavata. M'Coy. (PL XXIII. fig. 14). 



Sp. Ch. — Ovate, convex; length nearly twice the depth; one end broadly rounded, the other rounded, 

 but narrower ; valves smooth, a deep circular hollow in the middle of each, nearest the narrow end ; surface 

 smooth. 



At first sight this species resembles the C. impressa, M'Coy, but is distinguished by the larger size 

 of the depression in the valves, their more uniform convexity, and the total absence of tubercles. Length about 

 half a line. 



Cythere GiBBERULA. M'Coy. (PL XXIII. fig. 25). 



Sp. Ch. — Gibbous; back semicircular; ventral margin short, straight; ends very obtusely rounded; each 

 valve swelled in the middle into a large, undefined, rounded gibbosity. 



This species occurs in great numbers in the shale of some localities, it is chiefly remarkable for the large 

 liump or protuberance in the middle of each valve. Length half a line. 



Cythere HiBBERTii. M'Coy. (PL XXIIL fig. 15). 



Sp. Ch. — Oblong ; depth two-thirds the length ; ventral margin nearly as long as the shell, prominent ; 

 ends equal, subtnincate, rounded ; valves convex, compressed towards the ventral margin ; surface smooth. 



This is the largest species of Cjjthere I have met with in the Irish Palajozoic rocks, being frequently 

 upwards of a line in length. It resembles the C. injlata, M'Coy, but is much less gibbous, has a longer and 

 more prominent ventral margin, and less pointed ends ; the dorsal margin is also less convex in the present 

 species, giving it something of an oblong, square outline. 



This I believe to be the species figured by Captain Portlock as the Cypris Scotoburdigalensis of Hibbert, 

 but an examination of several Burdiehouse specimens induces me to give it, at least provisionally, a distinct 

 name. 



Cythere iMPEESSA. M'Coy. (PL XXIIL fig. 16). 



Sp. Ch. — Twice as long as deep, gibbous ; ends rounded, anterior slightly larger than the posterior ; ventral 

 margin nearly straight for about half its length, suddenly convex at the anterior end ; sides of the valves smooth ; 

 a deep, narrow depression close below the dorsal margin, at equal distance from each end, with a small tubercle 

 placed on its anterior edge. 



This species is very common in the slates and shales of several districts ; it is easily recognized by its 

 unequal ends, and, where the surface is preserved, by the deep, impressed puncture in the middle of the dorsal 

 margin, and its accompanying tubercle ; the valves are most gibbous at the anterior side of the depression. 

 Length about half a Hne. 



