133 Dr. J. E. Gray on the Division of Ctenobranchous 



F. Operculum spiral {rarely wanting) ; mantle and shell gene- 

 rally simple, sometimes with a rudimentary siphon and a canal in 

 front of shell ; et^^ sessile ; outer lateral teeth conical, curved. 



a. Eyes sessile, on outer side of tentacles. 



* Gill enclosed, in one or three lines on inner side of mantle- 

 cavity. 



Fam. 31. Littorinid^. Mantle edge simple or with only a 

 slight fold in front ; gills in two series ; shell free ; foot flat. 

 Assiminia, Littorina, &c. 



Fam. 33. Planaxid^. Mantle edge with a siphon and shell 

 with a notch in front. Planaocis, Quoyia, ? Litiopa. 



Fam. 33. MelaniadjE. Mantle edge torn, with a more or 

 less distinct siphon in front; gill of a single series of plates. 

 Rissoa, Skenea, Melania, Vibex, Faunus, Melanatria, Rhinoclavis, 

 Cerithium, Telescopium, Triphoris, Terebellum. 



Fam. 34. Vermetid^. Shell attached, irregular ; foot scarcely 

 fit for walking, dilated, clavate at the end. Vermetus, Serpuloides, 

 Siliquaria, &c. 



Fam. 35. ? VANicoROiDiE. Shell free; foot small, circular, 

 produced in front with a dilated membranous expansion on each 

 side ; operculum horny, ovate ; teeth ? Vanicoro. 



** Gills plumose, exposed ; lamina pinnate, spirally twisted, 



Fam. 36. Valvatid^. Operculum orbicular, spiral, many- 

 whorled. Valvata. 



b. Eyes sessile, on the head between or rather behind the base of 

 the tentacles. 



Fam. 37. C^cidjE. Shell subcyhndrical, arched ; apex deci- 

 duous, subspiral ; operculum circular, horny ; foot short ; teeth 

 ? Ccecum. 



Fam. 38, Truncatellid.e. Body and shell spiral ; foot very 

 short, roundish ; muzzle broadly 3-lobed : walks with its foot and 

 lips ; teeth ? Truncatella. 



G. Operculum none ; mantle and shell simple in front ; gills 

 in an oblique line across the mantle-cavity ; lamina elongate, linear, 

 pai'tly exposed ; eyes small, on the outer side of the base of the ten- 

 tacles. 



Fam. 39. Capulid^e. Foot folded on itself. Capulus, Hip- 

 ponix, Amalthia. 



Fam. 40. CALYPTRiEiD^. Foot expanded. Crypto, Galerus, 

 Crucibulum, Calyptra, Trochita. 



