200 Value of the Shells of Mollusca for the purpose 



but the family Muricidse embraces some species of Pleurotoma 

 and of Conus, whose shells are not easily distinguished by 

 family characters from some of the BuccinidsB. We are not 

 aware, however, that the soft parts could be distinguished 

 with any greater facility. This is an example of the grada- 

 tion of types into each other. 



The second order, Phytophaga, contains twenty-five fami- 

 lies, of which one, Atlantidse, is naked ; Tecturidse (=Lottia- 

 dae) is not distinguishable by ordinal characters of the shells 

 from Patellidse in another order ; the Littorinidse, containing 

 Solarium, cannot be distinguished from the Trochidae, which 

 contains Phillippia ; and Vermetidse and Dentalidse so closely 

 resemble certain Annelides, that it is doubtful ^whether any 

 family characters can distinguish them. The remaining 

 nineteen families may be distinguished by their shells with 

 greater or less facility ; the difficulties, where they exist, 

 arising from gradation of types, not from a want of correspond- 

 ence between the shells and the soft parts. 



The third order, Pleurobranchiata, contains five families, 

 with a remarkable intermingling of testaceous and of naked 

 genera ; yet the shells in each family are characterized with 

 the greatest facility, as may be seen from the well known 

 types of four of them, Bulla, Aplysia, Umbrella, and Carin- 

 aria, with which last Mr. Gray associates Argonauta. 



The fourth order, Gymnobranchiata, contains eight fami- 

 lies, of which two are testaceous, the Patellidse and Chitoni- 

 dsB, which are amply distinguished by their shells, except 

 that the former is too closely allied by its shells to Lottiadse, 

 as above mentioned. 



The fifth order, Pulmobranchiata, consisting of the air- 

 breathing Gasteropoda, contains twelve families, of which ten 

 are testaceous, and are easily distinguished by the shells, ex- 

 cept that Nanina is removed from the Helicidse and placed 

 with the Arionida3. 



