104 



Shell; heart-shaped, globose, ventricose, very inequilateral, concen- 

 trically ridged or striated, with the umboes distant and divari- 

 cately involuted ; hinge composed of two small 'cardinal teeth 

 interlocking, and one elongated lateral tooth ; ligament external, 

 drawn by the divaricating of the umboes into a bifurcation. 



In the Irish Channel and the surrounding ocean, ranging from Falmouth 

 to the Hebrides, and in the Mediterranean, dwells a bivalve mollusk par- 

 taking of the characters of Chama, Cardium, and Q/prina, and producing a 

 shell of comparatively large size, unique in these latitudes both in genus 

 and in species. Its chief peculiarity consists in the umboes divaricating 

 from one another, and turning symmetrically inwards. In the Chinese, 

 Japanese, and Philippine seas this very remarkable and beautiful type 

 again appears, not, however, in a large shell of inflated growth, covered 

 with a dingy olive epidermis, but in one of small dimensions, of which 

 there are four different specific forms elegantly concentrically ridged, de- 

 void of epidermis, and with a surface like carved and polished ivory. 



The animal of Isocardia is known only by observations made on the 

 large species, /. cor, by Poli, on Mediterranean specimens, and by the Rev. 

 James Bulwer, chiefly, on Irish specimens. The mantle is described as 

 completely lining the shell, the siphonal tubes being short, ciliated at 

 their orifices, and the foot muscular, triangular, and pointed.*" 



"It is capable, with the assistance of its foot, of fixing itself firmly in the 

 sand, generally choosing to have the umboes covered by it, and the orifices 

 of the tubes of the mantle nearly perpendicular. Resting in this position 

 on the margin of a sandbank, of which the surrounding soil is mud, at too 

 great a depth to be disturbed by storms, the Isocardia of our Irish Sea 

 patiently collects its food from the surrounding element ; assisted in its 

 choice by the current it is capable of creating by the alternate opening 

 and closing of its valves/' 



Species. 



1. cor, Linn. 3. Moltkiana, Chem. 5. vulgaris, Reeve. 



2. Lamarckii, Sow. 4. tetragona, Adams. 



* " On being placed in a vessel of sea-water the valves of the Isocardia gradually opened ; 

 and the feelers or ciliated fringe of the upper orifice of the mantle moved slowly as if in search 

 of animalcules. Having remained in this situation about ten minutes, water was ejected with 

 considerable force from the lower orifice, which till now had remained motionless. The expul- 

 sion of the water appeared to be affected by a sudden contraction of the muscles, because this 

 was never done without the valves nearly closing at the same instant. The animal appears to be 

 insensible both to sound and light, as the presence or absence of either did not at all interrupt its 

 movements : but its sense of feeling appeared to be very delicate. Minute substances dropped into 

 the orifices of the mantle instantly excited the animal, and a column of water strongly ejected 

 expelled them from the shell." — Bulwer in Zool. Journ., vol. ii. 



