INTRODUCTION. 51 



3. Calyptracea (plates, fig. 234 to 246). The patelliform 



shells of this family, although united by no other general 

 characters, are brought together by the characters of the 

 animals which produce them. The genera are Parmo- 

 phorus, Emarginula, Siphonaria, Fissurella, Pileopsis, 

 Calyptrsea, Crepidula, Ancylus. 



4. Bulleana (plates, fig. 247 to 253), contains the genera 



Bulla and Bullsea. 



5. Aplysiacea (plates, fig. 254 and 255). The genera 



Aplysia and Dolabella are both expanded, somewhat 

 flattened shells, with the apex placed at one extremity, 

 and slightly spiral. 

 G. Limacinea (fig. 256 to 263). Many of the animals (slugs) 

 are without shells ; some, as the Limax, or common 

 garden slug, have a slightly developed calcareous piece, 

 hidden beneath the mantle, and of others the shells are 

 scarcely spiral. The genera included in this family are, 

 Parmacella, Limax, Testacella, Vitrina. 



Order Trachelipoda. 



All the remaining spiral non-symmetrical shells are arranged 

 in this order, which is divided into the following families : — 

 1. Colimacea (plates, fig. 264 to 307). With the exception 

 of the few contained in the family of Llmacina, which 

 ought not to be separated from this order, the whole of 

 the land- shells are contained in this family, and although 

 it is difficult to notice any one character by which terres- 

 trial shells may be distinguished from others, few at all 

 conversant with the subject are liable to mistake them. 

 There is a general lightness and simplicity of form, 

 which, though not clearly definable, is generally under- 

 stood. The following distribution of genera by Lamarck, 

 is generally acknowledged ta require numerous modifi-. 



E 2 



