GASTEROPODA. PLEUROBRANCHID. 7 
marginibus attenuatis, anticé transversim plicatus, posticé liber. 
Testa calearea, spira subdepressa. 
Fam. 4. Butitap#. Branchie dextre, sub pallii dila- 
tatione tectee. Pallivm super branchias dilatatum. Tentacula 
nulla. Zabivm superius supra dorsum productum, lateraliter 
dilatatum, discum efformans. Pes latissimus, cum labio infe- 
riore penitus coalescens, lateraliter valdé dilatatus latera am- 
plectens, posticé ultra corporis medium productus. Testa 
involuta, spira seepius abscondita. 
Branchie situated on the right side of the back, or on the 
right side of the animal, and covered by a reflection of the 
mantle. Shell external, internal, or entirely wanting. Anus 
on the right side. Sex hermaphrodite. 
All the genera of this Order are marine. 
FAMILY I. PLEUROBRANCHIDZ. 
Branchie inter pallu et pedis junctionem affixe. Testa 
fragilis. Tentacula duo involuta, labialia. Pes marginibus 
dilatatis. Pallivm undique dilatatum, testam tegens. Anus 
et Sexuum organa anteriora, dextra, 
Synopsis Generum. 
Os tubulosum. Labivm crassum, supra ac 
infra lamella simplici instructum : lamella 
superior major: inferior ex pedis reflec- 
tione efformata. Pallium integrum pede 
multo latius. Branchie elongate. Pes 
posticé angustior, integer. Testa calca- 
rea, oblonga, aperta ....... . 10. CLEANTUs. 
Os reconditum, lamella cr a emacuks eee 
anticé cornuta, superne instructum. Pal- 
lium pede multo angustius, anticé et pos- 
ticé emarginatum. Branchie valde elon- 
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