20d Conchology, 



ribs which are found on the outer surface of other shells; while 

 the whole of the internal surface is smooth and poUshed. In bi- 

 valve shells, which exhibit this structure, the whole anterior sur- 

 face of the animal is grooved or channelled in the same way; and 

 from this, the shell derives its shape and structure. In these 

 shells it may be observed, that it is only the anterior margin that 

 is grooved on the internal surface; because in the progress of the 

 growth of the animal, that part of the body which presents a 

 smooth, equal surface has advanced, and nearly filled the whole 

 of the shell; and the testaceous matter secreted from this part of 

 the body being deposited on the grooves, channels, or striae, 

 which were formed when the anterior part of the body occupied 

 that part of the shell, fills them up completely, and leaves the sur- 

 face quite smooth and poHshed. New additions being made to 

 the shell as the growth of the animal requires it, the smooth sur- 

 face of the body advances forward, and fills up with secretions 

 what is now^ grooved; while the new part of the shell which cor- 

 responds to that part of the body which has an unequal surface, 

 only presents this appearance. It is ih this way that the ribs or 

 grooves are formed in different species of ostrea, cardium, and 

 other bivalve shells. 



But there is a pecuHarity of structure in a species of cockle, 

 the white fluted or ribbed cockle, Cardium costatum, which seems 

 more difficult of explanation in its mode of formation. 



The ribs of this species are not only of the usual structure of 

 other species of ribbed or grooved shells, but are particularly dis- 

 tinguished by having them hollow. The whole number of the 

 ribs amounts to about eighteen on each valve, of which the eleven 

 exterior ones are of a triangular fornij of about three lines high, 

 and hollowed through their whole length, from the beak to the 

 margin of the valves. To have a distinct notion of the formation 

 of these hollow, triangular ribs, it is necessary to conceive, that 

 the margin of the anterior parts of the animal, is deeply channel- 

 ed or grooved; and when this part is in contact with the recent 

 shell, the ribs or elevations are formed, and are then open to the 

 internal surface of the shell; but the posterior part of the body 

 being hard and smooth, never comes in contact with the excavated 

 part of the ribs. On the contrary, as the testaceous matter is 

 excreted from this part of the body, it is deposited on that part 

 of the internal surface of the shell which it touckes, stretches 

 across the deep grooves, and forms the third and interior side of 

 the triangular ribs. Thus it appeal's, that spines, tubercles, and 

 all other protuberances on the surface of bivalve shells, owe the 

 peculiarity of their form and shape to the peculiar structure of dif- 



