264 Prof. C. B. Adams on the 



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 Cari- 

 naria, with which last Mr Gray associates Argonauta. 



The fourth order, Gymnobranchiata, contains eight fa- 

 milies, of which two are testaceous, the Patellidse and Chit- 

 onidse, which are amply distinguished by their shells, except 

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

 as above mentioned. 



The fifth order, Pulmobranchiata, consisting of the air- 

 breathing Gastropoda, contains twelve families, of which ten 

 are testaceous, and are easily distinguished by their shells, 

 except that Nanina is removed from the Helicidae, and placed 

 with the Arionidge. 



Although in the class Conchifera, the shells do not furnish 

 good ordinal characters for the five orders of Mr Gray's list, 

 yet all of the 42 families which are distributed through these 

 orders may be only distinguished by their shells. Also the six 

 families of the Brachiopoda, and those four of the Pteropoda, 

 which are testaceous, are easily distinguished by their shells. 



Thus, of the 95 testaceous families of Mollusca, 88, or 

 more than 92| per cent, are distinguishable by their shells. 



For the generic value of the shells we may take the results 

 in Mr Gray's list, which is remarkable for its multiplication 

 of genera ; 810 recent genera are recognized. Of these 152 

 are naked, and 648 are testaceous.* Of the latter the shells 

 of two genera are said by Mr Gray to be indistinguishable 

 from certain Annelides ; and of seven others it is said to be 

 impossible to characterize them by the shells only. This list, 

 although small, is susceptible of reduction ; for M. Deshayes 

 has pointed out the difference between the shellsof Verme- 

 tus and. of Serpula ; and Dr Gould has discovered generic 

 characters in the shells of Lottia, distinguishing them from 

 Patella. Of 648 testaceous genera, 641 therefore may be dis- 

 tinguished by their shells. This amounts nearly to 99 per cent. 



Although the soft parts of 5000 species of Mollusca, in- 

 cluding both the naked and the testaceous species, are 

 known, we are not aware of the existence of a single testa- 



* Here there is a mistake. — Edit. 



