THE GASTROPODA 151 
apophysis. .Genus—Hydrocena, Parreys; from Dalmatia. Faminy 23. 
PROSERPINIDAE, Fischer. Differs from the two last families in not 
having an operculum. Genus—Proserpina, Gray ; from Central America. 
ORDER 2. Pectinibranchia. 
These are Streptoneura with a somewhat concentrated nervous 
system ; without a labial commissure, except in Paludina and Ampul- 
laria. The nerve-collar is situated behind the buccal bulb, except in 
Ampullaria. ‘There is a single well-differentiated, independent, and 
often pectinated osphradium. The eye is always closed, and the 
internal cornea (pellucida) is extensive. Each otocyst contains a 
single otolith, except in some forms of 'Taenioglossa devoid of a pro- 
boscis, e.g. Paludina, Valvata, Ampullaria, Cyclophorus, Typhobia, 
Bythoceras, Nassopsis, certain Cerithiidae, etc. The central tooth of 
the radula is single or absent. There is no longer any trace of 
bilateral symmetry in the circulatory, respiratory, and excretory 
organs, the topographically right half of the pallial complex having 
completely disappeared. The heart has only a single auricle—that 
of the morphologically right side—and is not traversed by the rec- 
tum. The ctenidium is monopectinate and attached to the mantle 
throughout its length, except in Adeorbis and Valvata, the latter 
genus being the only Pectinibranch with a bipectinate ctenidium. 
The single kidney usually opens directly by a slit-shaped aperture 
(but exceptionally by a ureter in Paludina, Cyclophorus, and Valvata), 
and never serves for the passage of the sexual products. The 
gonad always has a separate orifice of its own. The male gencrally 
has a penis (Fig. 44, A, A). 
The Pectinibranchia are divided into two sub-orders—Taenio- 
glossa and Stenoglossa. 
Sup-OrpDER 1. TAENIOGLOSSA. 
In these Pectinibranchs the radula has normally three teeth on each 
side of the median tooth, viz. one lateral and two marginals (Fig. 2, B; 
74, B). The stomatogastric ganglia are situated behind the buccal mass, 
and are united to the cerebral centres by long connectives which are in 
part recurrent and deeply situated. The salivary ducts, when sufficiently 
long, traverse the nerve-collar. The oesophagus is nearly always devoid 
of an unpaired gland. Usually there is neither a proboscis nor a siphon. 
The sub-order includes two distinct groups or tribes, which are 
respectively creeping and swimming forms, namely, the Platypoda and 
Heteropoda. 
TRIBE 1. PLATYPODA. 
Normal Taenioglossa, but slightly modified, and of creeping habit. 
The foot is flattened ventrally, at all events in its anterior part (Strom- 
bidae). The otocysts are situated close to the pedal nerve-centres. Acces- 
