420 EXTERNAL FORM OF SHELLS. 



like the Pectines, Carclia, &c., they are frequently variously 

 ribbed, a circumstance which often takes place in the Com- 

 mon Anomiae ; and if attached to a Dolium, as in a specimen 

 in the collection belonging to Mrs. Mauger, they even ex- 

 hibit on their own surface the alternate broad and narrow 

 ribs of that shell. In specimens of Crepidula adunca at- 

 tached to Trochus doliarius (and inhabiting the same locality, 

 they are not unfrequently so attached), the convex part of 

 the former is marked with the ribs of the latter. Shells 

 which are ribbed from this cause are, however, easily dis- 

 tinguishable from those which are naturally ribbed, the ribs 

 in the former generally extending across or along the shell, 

 and not radiating from its apex or nucleus, as in all shells 

 the natural character of which is to be ribbed. In those 

 which adhere to ribbed shells by the foot of their animal 

 (as in most of the univalves), and are therefore capable of 

 being moved from place to place, the young animal may 

 have lived on a smooth surface, and have had a smooth shell ; 

 and may have moved, during its growth, to a ribbed body, 

 producing ribs on the latter formed part of its shell, or vice 

 versa. In a specimen of Crepidula adunca, for example, in 

 the British Museum, the upper half of the shell is smooth, 

 and the lower half ribbed ; and I have seen specimens, on 

 the contrary, in which the apex was ribbed and the base 

 smooth. This change of form has, however, misled some 

 conchologists, for Dr. Bronn, of Heidelberg, has founded a 

 genus under the name of Brocchia, on a specimen of Capu- 

 lus, which had acquired a ribbed surface in consequence of 

 having been attached to a Pecten or to some other radiated 

 shell. 



These alterations of form and surface are always most 

 distinct in univalves and in the upper valve of bivalves. In 

 the latter case, the edges of the upper valve being produced 

 beyond those of the under, they are immediately moulded 

 on the surface of the substance to which the shell is at- 

 tached, whilst the under valve simply covers it over. This is 

 well illustrated in the unique specimen of Hinnites gigantea 

 in the collection of the British Museum, which must have 

 been attached to some marine body having a Serpula grow- 

 ing upon it. There is merely an irregular convexity in the 

 inner part of the under valve, but on the outer surface of 

 the free valve is to be observed a representation of the whole 

 form, and of almost the entire surface of the Serpula, in 

 consequence of the edge of that valve, during each deposi- 

 tion of shelly matter, having rested on the worm-shell. In 

 the collection of Mr. Lincolne is a specimen of an oyster 



