THRACIA. 233 



and remarkably small ; the umbones are not very promi- 

 nent; and the hinge consists of a strong and rather narrow 

 oblique triangular plate, situated close under the beaks on 

 the posterior side, which is more or less deeply hollowed at 

 the top for the reception of the cartilage, and whose free 

 extremity curves upward in both valves, but is only elevated 

 above the surface in the right one. By careful examination 

 there is to be perceived in the upper part of its front edge 

 an emargination adapted to the size of its most minute 

 moveable ossicle. 



This bivalve never attains to any considerable size ; the 

 largest specimen we know of not exceeding three quarters 

 of an inch in length, and not quite half an inch in breadth ; 

 ordinary examples are one-third less, but the breadth often 

 equals the length. 



The animal of this species, according to the notes of Mr. 

 Clark and Mr. Alder, appears to differ from its congeners 

 but slightly. It is more or less suborbicular in shape, 

 entirely white, and has the mantle closed, except a small 

 aperture for the passage of a small, oval, linguiform, fiat 

 foot ; the tubes are rather short, and slightly more united 

 at their origins than in the other British Thraci(E ; the 

 branchial siphon is often extended more than half an inch, 

 whilst the anal remains quiescent ; previously to its being- 

 retracted, it is always globularly inflated at its extremity, 

 which inflation increases in size until its near approach to 

 the margin of the shell, and then suddenly disappears ; 

 when inflated, all signs of the cirrhi are lost, and they only 

 become visible when the tube is at rest. This operation 

 seems to be performed for the purpose of ejecting water 

 and rejectamenta with greater force (Clark). The branchiai 

 are large and brown, one lobed on each side ; the labial 

 tentacles are nearly equal and pectinated. 



VOL. 1. H H 



