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marine tribes, the Turbo variabilis might be selected as a specimen of 

 simple convolution, the elongated modification being represented by the 

 Turritella or Terebra, and the depressed by Solarium or Kotella. The land 

 species offer the most extreme modifications of depressed growth, because 

 the visceral parts of the herbivorous tribes may very possibly be restrained 

 within a more limited area than the carnivorous kinds ; they have no occa- 

 sion for the armature of teeth and rasped plates required for the assimilation 

 of animal food, nor is the digestive apparatus of so complicated a character. 



The carnivorous tribes have no such limitation of growth as is found in 

 the Caracolla, their range of habits furnishes them with more vigour, and 

 their calcifying energies are exercised in many instances to an excess winch 

 is truly astonishing. A ponderous massive shell is often found to be the 

 production of an animal occupying but a very limited portion of it. The 

 shells of the terrestrial species have no external ramifications and very little 

 variety of sculpture, whilst those of the marine kinds are ornamented with 

 ribs, tubercles, laminae, spines, and fronds. The first departure from the 

 fluted LentaUum, or the smooth tube of Turritella, is presented in the Sca- 

 laria, where the lip of the aperture is reflexed in its earliest stage of deve- 

 lopment ; the course of the tube proceeds with the reflexed margin remaining 

 on its circumference ; and tins operation is continued at intervals until the 

 shell and its inhabitant arrive at maturity. This is the simple plan upon 

 winch the ornamental structure of all shells is developed. To take a more 

 complicated example, let us examine the growth of the Murex. Not only 

 is there a periodical reflexion and thickening of the edge of the tube, which 

 in this genus has an ovate or pyriform area, but certain calcifying filaments 

 or processes are exserted from the edge of the mantle, capable of produc- 

 ing most elegantly formed spines and fronds. As soon as this architectural 

 border is finished the filaments are withdrawn, and the tube pursues its 

 regular growth until they are again exserted for a similar purpose. These 

 borders, technically called varices, thus encircle the tube at intervals, and 

 are supposed to indicate seasons of rest ; the lip of the shell being probably 

 thickened in tins manner for protection during a period of relaxation. The 

 varices are formed at various intervals ; in Triton only two or three occur 

 during the entire growth of the shell, in Uanella one is deposited on every 

 half whorl, in such a manner as to range obliquely one under the other, 

 and in Murex they occur tliree or more times on every whorl. In Harpa 

 and other genera of the family Purpurifera, the entire shell is formed of a 

 close succession of marginal borders like the varices of the Canalifera. 



There are many varieties in the tubular growth of the shell besides those 

 above enumerated ; in the Cone, for example, the tube is of a longitudinally 

 compressed form, winding upon itself almost on the same vertical plane, 

 but as the different modifications of form and sculpture will be treated of 

 under the different genera, it only remains to notice the varieties of the res- 



