1 86 



THE GASTROPODA 



transformed into a branchia. Genera Ancylus, Geoffroy ; no pulmonary 

 cavity ; animal dextral or sinistral ; visceral commissure shortened ; shell 

 without internal septum; British (Fig. 176). Latia, Gray ; a pulmonary 

 cavity ; visceral commissure long ; shell with a posterior internal septum ; 

 from New Zealand. Gundlachia, Pfeift'er. FAMILY 11. PHYSIDAK, Dall. 

 Visceral mass and shell sinistrally coiled ; shell thin, with a narrow 

 aperture ; tentacles cylindrical ; no inferior pallial lobe. Genera 



Physa, Draparnaud ; shell oval, 

 partly covered by the edges of the 

 mantle, which are divided into 

 angular tags ; British. Aplexa, 

 Fleming ; shell with a pointed 

 spire ; edges of the mantle not 



t 



nmA 



divided and very slightly re- 

 flected over the shell ; British. 



PIG. 175. 



Bulinus tabulatus, ventral aspect. 

 br, pallial extrapulmonary gill ; co, 

 heart ; o, mouth ; p, foot ; pa, mantle ; 

 pns, pneumostome ; te, tentacle. 



FIG. 176. 



Ancylus fluviatilis, dorsal 

 view. To the left, the head 

 with the two cephalic ten- 

 tacles. (From Lankester, after 

 Reeve.) 



SUB-ORDER 2. STYLOMMATOPHORA. 



Pulmonata with two pairs of tentacles (except the Janellidae and Vertigo, 

 which have only a single pair) ; these tentacles are invaginable, and the 

 eyes are borne on the summits of the posterior pair. The male and 

 female genital orifices open into a common vestibule except in the 

 Ditremata (Vaginulidae and Oncidiidae). A suprapedal gland is present 

 in nearly all the groups. With the exception of Uncidium, there is no 

 longer a veliger stage in the development ; the embryo is often furnished 

 with a contractile pedal vesicle (Fig. 1 1 7). 



The Stylommatophora may be divided into four tribes : the Holognatha, 

 Agnatha, Elasmognatha, and Ditremata. 



TRIBE 1. HOLOGNATHA. 



Jaw simple, without a superior appendage. 



FAMILY 1. SELENITIDAE, Fischer. Radula with elongated and 

 pointed teeth, like those of the Agnatha ; a jaw present, Genera 

 Selenites, Fischer ; shell external, depressed, widely umbilicated. Plutonia, 

 Stabile ; animal limaciform, with flattened internal shell and a posterior 

 pulmonary aperture. Triyonochlamys, Bottger ; no shell. FAMILY 2. 

 ZONITLDAE, Pilsbry. Shell external, smooth, heliciform or flattened ; 

 radula with pointed marginal teeth. Genera Zonites, Montfort ; 

 shell depressed, wholly external; British. Ariophanta, Desmoulins ; 



