240 ARCADE. 



very distant beaks. The umbones are prominent. The 

 interior is of an uniform white, and its margin is quite free 

 from crenation ; the hinge-margin increases greatly in 

 breadth at both ends, its lower edge is incurved, and the 

 teeth, which are rather numerous, enlarge considerably and 

 nearly equally at both ends. 



Our largest specimen only measures three-quarters of an 

 inch in length and half an inch in breadth. 



The animal, according to Mr. Clark, is oblong, thick, 

 body white, mantle pale red towards the apex, entirely 

 open, having the under surface of the ventral range marked 

 on a very pale yellow ground with irregular fleshy blotches, 

 and on the upper for some little depth with a sand-like 

 rusty brown margin, and a darker interrupted bordering 

 line near the slightly dentated edge. There are no equi- 

 distant black points or ocelli as in Area tetragona. The 

 foot is white with a deep fissure at its bend, and is usually 

 provided with a green byssal membrane by which it is 

 attached to various substances, from which, however, the 

 animal can freely cast itself oiF and spin a new byssus. 

 Even when fixed the foot can be protruded for a consider- 

 able length, and it then appears fleshy and tapering, and of 

 a pure white colour. On each side of the body are two 

 very thin narrow symmetrical pale brown branchiae, gradu- 

 ally tapering on each side, uniting around the mouth, and 

 forming, instead of palpi, a double lamina or lip. 



This little Ark is, in the main, a southern species. It 

 occurs at Herm and elsewhere in the Channel Islands, 

 profusely attached to dead bivalves (S. H.) ; Port- 

 land Island in fifteen fathoms, occupying the crevices of 

 shells, Penzance in twenty fathoms (M' Andrew and 

 E. F.) ; Shelborne, Sandwich, Whitesand Bay, Ufracombe, 

 Fishguard in twenty fathoms, and Manorbcer, in Pern- 



