THE SHELL. 17 



moves forward, leaving the small tip of the operculum free, and 

 useless to the animal, and, therefore, not necessary to be repro- 

 duced when the operculum is reformed in the adult age of the 

 animal."* 



The Shell. 



All prosobranchiates (and nearly all mollusks) are provided 

 with an external shell, a dwelling place and a citadel combined, 

 the hardness and durability of which, as Keferstein remarks, 

 " supplies us with the best means of knowing the animal ; in- 

 deed, in many cases, it is the only part known, and was formally 

 the only part valued and preserved in collections. Although 

 the animal itself offers more weighty and striking characters for 

 the separation of the higher groups, yet having learned the close 

 relationship existing between shell and animal, we find therein 

 ample justification for attaching especial importance to the shell 

 in a systematic point of view." 



We have already shown how the shell is produced by the 

 mantle. 



The form of the shell is throughout regular, and is normally a 

 cone curved into a spiral, and descending in a screw-like manner 

 from the apex or initial whorl to the aperture. Nothing can be 

 more beautiful than the regular geometrical progression of the 

 growth of a shell or the certainty with which each species and 

 genus grows in its normal pattern, although these modes vary 

 among themselves so widely : thus we have the simple depressed 

 cone of the Patella, all aperture and no spire, and from it every 

 gradation from the Haliotis, almost equally depressed and broad, 

 the result, however, of a very rapidly enlarging spiral, to the 

 long, many-whorled Turritella, or the Vermetus, which is a 

 Turitella partially unrolled into a simple long tube : the opposite 

 of the Patella. The whorls of a spiral shell are, in most cases, 

 closely wound around its axis, and, therefore, most part of their 

 surface is in contact, each whorl partially covered and concealed 

 by its successor ; and where the axis does not lengthen by the 

 obliquity of the spiral, we find, as in the cone and Cypraea that 

 the shell only shows externally its last whorl, with, perhaps, a 



* Dr. J. E. Gray, Proc. Zool Soc., 100, 1854. 

 3 



