ABSUKI'TIUN OF Till-; SHELL. 437 



species. It appears nioveover to be of little importance how 

 great may be the hardness, or what may be the structure, of 

 tlie shell on whicli they fix ; all yielding with equal readi- 

 ness to their abs()r])ing powers. It is not vniusual to find 

 holes thus produced nearly a quarter of an inch in depth in 

 the very hard external coat of the larger Turbines; and 

 similar depressions are found in Purpura*, Strond)i, l''is- 

 surellac, Chitones, Patellye, &c. 



The animals of Siphonaria, Patella, and an allied genus 

 (Lottia), which appears to be peculiar to the coast of South 

 America, have the sanie faculty, ])ut in a less degree, and the 

 cavities formed by them are destitute of the horse-shoe- 

 sliaped ridge. The depressions })roduced by the Siphonaria 

 and the Chitons have, however, an unequal groove round 

 their margin, which is largest and deepest on one side, occa- 

 sioned probably by the shell being generally raised on the 

 opposite side to admit of the access of air to the branchiae. 

 The Patella cochlea is often found at the Cape of Good 

 Hope, where it lives almost exclusively attaclied to a large 

 species of the same genus, on the surface of which it forms 

 a flat disk, exactly the size of its mouth. To form these 

 flat disks (of which there are so generally two, one on each 

 side of the apex of the large Patella, as almost to form a 

 character of the species), and to assist in the increase of its 

 size, the animal appears also to absorb any coralline or other 

 similar substance with which the larger shells are abundantly 

 covered. The ccmnnon Patella of our own coast, when hnig 

 adherent to another shell of its own s[)ecies, to chalk, or to 

 old red sandstone or limestone, also forms for itself a deep 

 cavity of the same shape as its shell, and evidently produced 

 by the dissolution of the surface to which it is afhxcd. 



The animals of the several species of Vermetus, especially 

 of that called by Daudin Spiroglyphus, have the faculty of 

 producing by absor])tion a groove in the surface of many 

 very hard shells, such as the Trochi, Ilaliotides, and Fissu- 

 rella; ; which groove they cover with a calcareous deposit, 

 and thus form it into a tube. The history of Spirogly])hus 

 is altogether peculiar: the young animal, when first hatched, 

 is covered with an ovate regular spiral shell, consisting of a 

 whorl and a half, and in appearance very like the yovmg shell 

 of Magilus, with which, indeed, its aflinity is very striking; 

 it soon attaches itself to the surface of a shell, in which it 

 commences the formation of a canal, narrow and shallow in 

 the first instance, but becoming deeper and wider as the 

 animal increases in size. Both the canal and its shelly cover- 

 ing retain for some time the regular discoidal spiral form. 



