94 UNIVALVES. 



according to the arrangement of these ornaments. 

 In one division the shells are distinguished by 

 having two varices in each whorl, which are con- 

 tinued in a longitudinal series on each side up 

 to the apex, giving them a flattened appear- 

 ance on the front and back ; the genus which 

 they now constitute is called Ranella* Frog, 

 from the resemblance of the shells to that animal. 



The shells of another division f are distin- 

 guished by three rows of varices, sometimes ex- 

 panding into elegant foliations or lengthened 

 spines, these varices are so arranged that they 

 form three longitudinal raised lines taking rather 

 an oblique inclination towards the apex. It is 

 evident that the habits of the mollusca of these 

 two sets of shells must be different, for the situa- 

 tion of the varices of the latter proves that a 

 smaller portion is added to the abode at each 

 epoch of its growth than in the Ranella. 



A third division J of this genus includes shells 

 ornamented by varices, not disposed in a con- 

 tinued series as in the other two classes. This 

 difference in the disposition of these protuber- 

 ances is occasioned by the piece which the 

 animal adds to its shell being more than half a 

 volution. The varices are consequently few, 

 and appear scattered over the shell. 



* Plate IV. fig. 2. t Plate IV. fig. 1. 



$ Plate IV. fig. 3. 



