188 CORBULIDiE. 



late the probabilities of geographical distribution. About 

 ten years have now elapsed since the date of its publica- 

 tion (1838), during which period no second example has 

 been discovered : should no further specimens then be 

 procured upon our coast, the finding of the only recorded 

 one must be attributed to some such incident as the im- 

 bedding of the living mollusk in the tangled roots of some 

 fucus clinging to the oysters or cirrhipedes so wont to con- 

 gregate upon ship timber, in a foreign port, and the sub- 

 sequent detachment of the sea-weed, either in the process 

 of careening, or perchance by the breaking up of the vessel 

 itself. 



As but a single specimen of this shell has ever been 

 taken upon the coasts of Great Britain, the following de- 

 scrij)tIon must rather be regarded as the portraiture of an 

 individual than as a specific definition ; since the latter 

 may not be depended upon, unless based, not merely upon 

 the characters present in one example, but from the aggre- 

 gate of features existing in several, which remaining un- 

 changed amid the many modifications of form, colour, or 

 sculpture to AAdiich every shell is liable, may reasonably be 

 supposed to be the permanent characteristics. 



The shape is oblong-elliptic, and very nearly, if not 

 quite, equilateral ; the valves, which are opaque and not 

 very solid, although rather ventricose at the umbonal 

 region, are but moderately convex upon the whole, and 

 difter but little from each other in either size or pro- 

 fundity ; the right one, however, very slightly overlaps 

 the other below, and very slightly projects beyond it 

 above. There is an appearance of erosion at the umbones, 

 which prevents the accurate determination of this latter 

 point. The surface is devoid of sculpture in both valves, 

 unless we reckon a few antiquated lines of growth as such ; 

 it is dull white, and is covered with a lustreless skin of 

 squalid white, becoming of an ashy ferruginous cast (pos- 

 sibly a mere extraneous coating), chiefly in the vicinity of 

 the lower margin of the lesser valve, where it becomes 

 more or less distinctly wrinkled in a concentric direction. 



