14 BRITISH MARINE TESTACEOUS MOLLUSCA: 
and Pars-pectinibranchiata. The first four carry branchize, 
or respiratory network in particular cavities of the back and 
neck ; they all swim or float, except the Limacide and Heli- 
cide. 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 Pleurobranchide are fully noticed hereafter, 
and the Aplysiade are too well known to require any remark. 
The Bullide 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 Bullide, is the 
testaceous hard gizzard, in respect of which our opinions are 
not changed, of its bemg im 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 Bullea 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 Bulla and Bullea that we have observed 
