14 BEITISH MARINE TESTACEOUS MOLLUSCA: 



and Pars-pectinibranchiata. The first four carry branchiae, 

 or respiratory network in particular cavities of the hack and 

 neck ; they all swim or float, except the Limacidts and Heli- 

 cidce. It is necessary now to state why the Pteropoda do not 

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

 identical with the hermaphrodite Pectinibranchiata of this 

 division, that I have preferred placing them in conjunction, as 

 they have the same sexual characters, rather than 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 Ce- 

 phalopoda, an equally inconsistent situation. The term Ptero- 

 poda, inferring that the foot, or locomotive, is formed like 

 wings, and fixed more or less around the neck, does not, we 

 think, militate against these animals being considered modi- 

 fied Gasteropoda ; in like manner as the Trachelipoda of La- 

 marck, that have them under the neck, which is only a modi- 

 fication of the foot, that in many tribes occupies the entire 

 length of the body. The wings of the Pteropoda are little 

 more than broad membranous extensions of the anterior lateral 

 portions of the foot to assist natatory locomotion on its natural 

 element. The Pleurobranchida are fully noticed hereafter, 

 and the Aplysiada are too well known to require any remark. 

 The Bullidce are a difficult family, many of the animals being 

 very minute, and some have never occurred to any naturalist. 

 We have preferred depositing the Velutina otis of authors, 

 Mr. Gray's Otina, ad interim in this family, rather than 

 amongst the Conovuli ; at the same time, it is as probable it 

 may belong to them, especially if it has not a gizzard. The 

 distinguishing character of most, if not all, the Bullidce, is the 

 testaceous hard gizzard, in respect of which our opinions are 

 not changed, of its being in some shape or other the invariable 

 concomitant of this group ; but we admit that in the minuter 

 species it is so exceedingly small as easily to escape attention; 

 we have found it so in Bullcea punctata, and only succeeded in 

 finding it after several examples had been examined. It is 

 possible that some of the species may have strong coriaceous 

 gizzards, but in every Bull a and Bulhea that we have observed 



