THE GASTROPODA 173 
Faminy 2. PNEUMONODERMATIDAR, Gray. Shell and mantle absent ; foot 
shorter than the visceral mass, and with a much-reduced ventral surface ; 
parapodia highly developed and fin-shaped ; pharynx evaginable, with 
suckers. Genera—Dewiobranchaca, Boas ; suckers independent ; no terminal 
posterior branchia. Pnewmonoderma, Cuvier ; suckers united on two lobes ; 
a quadriradiate terminal branchia. Spongiobranchaea, dOrbigny; a 
simple annuliform terminal branchia.  Schizobrachiwm, Meisenheimer ; 
acetabuliferous appendages ramified. Famity 3. CLIoNopsIDAE, Costa. 
No buccal appendages or suckers ; a very long evaginable proboscis ; a 
quadriradiate terminal branchia. Genus—Clionopsis, Troschel. Famity 4. 
NoToBRANCHAEIDAB, Pelseneer. Posterior branchia triradiate. Genus— 
Notobranchaeu, Pelseneer. Faminy 5. THLIPTODONTIDAE, Kwietniewski. 
Head very large, not marked off from the body ; neither branchia nor 
suckers ; fins situated near the middle of the body. Genus—Thliptodon, 
Boas. Famity 6. Cxuionmar, Gray. No branchia of any kind; a 
short evaginable pharynx, bearing paired conical buccal appendages or 
“ cephalocones.” Genera—Clione, Pallas. Paraclione, Tesch. owlerina, 
Pelseneer. Famity 7. HatopsycuIDA®, Pelseneer. No branchia ; two long 
and branched buccal appendages. Genus—Halopsyche, Boas (Fig. 156). 
The last six families form the group formerly known as the “ Gymno- 
somatous Pteropoda,” characterised by the absence of the mantle and shell, 
the reduction of the ventral surface of 
the foot, and the fin-shaped parapodia 
placed at the anterior end of the body. 
They are all pelagic. 
TRIBE 3. PLEUROBRANCHOMORPHA. 
In these Tectibranchs there are 
two pairs of tentacles. The foot is 
devoid of parapodia, There is no 
pallial cavity, but there is always a 
single ctenidium situated on the right 
side and occupying the space between 
the mantle and the foot. The genital 
duct is diaulic, without an open 
seminal groove ; the male and female 
apertures are contiguous. The vis- 
ceral commissure is short, and re- 
sembles that of the Nudibranchs in 
showing a tendency to the fusion of 
the supra-intestinal and sub-intestinal : 
with the pleural ganglia, and the Se ICES epopelirs ek eR eg a : 
concentration wf all the ganglia on 11, foot: IV, gill; Y, position of the anus, 
: itle ; VI, orifice of Bourne’s 
the dorsal side of the oesophagus. prebranchial gland; VII, genital (herma- 
Fasney 1. Teropmiman, Mazza Dupive) artes; Vir, the sued anterio: 
relli. Shell external and conical ; the 
anterior tentacles form a frontal veil; the ctenidium extending only over 
the right side; a distinct osphradium. Genus—Tylodina, Rafinesque ; 
Fic. 157. 
