British Marine Testaceous Mollusca. 477 



of the branchise, with Teredo, the last family of the Acephala. 

 The Chitons approach nearest to the term Cyclobranchiata. The 

 Patellae and Calyptrseadse, in our method, are cervicobranchiate 

 patelloid forms, with a single non-symmetrical branchial plume. 

 The Fissurellidse are in the same cervicobranchiate category, as 

 well as the Haliotidse, but differ from the first patelloid group in 

 having two symmetrical branchial plumes. 



As most of these animals will be noticed in the descriptive ac- 

 counts of the species, it is unnecessary to make further remarks, 

 except to observe, that Haliotis does not inhabit this side the 

 British Channel; but M. Cuvier has figured and given an ana- 

 tomy of it amongst his memoirs. 



Fourth Division. 



f cryptibranchiata. 

 Gasteropoda < pleurobranchiata. 

 \jpulmonifera. 



In this division there is an important advance in sexual ar- 

 rangement ; pure hermaphroditism is abandoned, and that of mu- 

 tual congression has succeeded. The families are the Pteropo- 

 didse, Pleurobranchidse, Aplysiadse, BuUidse, Tornatellidse, and 

 the Pulmonifera, which carry the branchise in particular cavities 

 of the back and neck : they all swim or float except the Hehcidse. 

 It is necessary now to state why the Pteropoda do not constitute 

 with us a separate class ; their anatomy is so nearly identical with 

 that of these families that I have preferred placing them in con- 

 junction, rather than with the sexual characters of hermaphro- 

 ditism with congression, letting them remain in a false position, 

 between groups of the Acephala and Gasteropoda, both of which 

 are strict hermaphrodites, or intercalating them between the 

 bisexual Gasteropoda and Cephalopoda, an equally inconsistent 

 situation. 



The term Pteropoda, inferring that the foot or locomotive or- 

 gan is formed hke a wing and fixed more or less around the 

 neck, does not, I think, militate against these animals being con- 

 sidered modified Gasteropoda, in like manner as the Trachelipoda 

 of Lamarck that have them under the neck, which is a modifi- 

 cation of the foot, which in many of these animals occupies the 

 extent of the body : all these creatures swim and creep like most 

 of the Gasteropoda. 



The Plem-obranchidse are noticed hereafter, and the Aplysiadse 

 are too well known to require any remark. The Bullidse are a 

 difficult family, many of the animals being very minute, and 

 some have never occurred to naturalists. We have preferred 

 depositing the Velutimt otis of authors, Mr. Gray's Otina, ad 



