156 PROCEEDINGS OP THE MALACOLOGICAL SOCIETY. 



from the body. A short distance behind the ends, however, the 

 filaments are connected by an interfilamentar blood vessel (the 

 marginal vein carrying blood from the oral sinus venosus), and this 

 vein (m) is attached along the 'body side. From this vessel there rise 

 small septa running backwards all along the inside of the filaments, 

 and also towards the front end of each filament a small septum arises. 

 The latter septa are all combined with a thin membrane, which 

 thus covers the inside of the front part of the gill turned towards 

 the foot. Behind the foot, in about the middle of the gill, 

 the marginal vessel is detached from the body and joined with the 

 corresponding one of the opposite gill (at e) ; thus, behind the foot, 

 an interbranchial septum is formed including the blood vessel. The 

 opposite margins of the gills (the ends of the filaments) end freely 

 without being mutually connected. At the end of the gill the septum 

 is attached to the mantle beneath the anal siphon, and the vein passes 

 into the axial vessel which brings the blood into the heart. 



In fertile specimens the posterior septa are highly developed by 

 inflation and connexion into incubatory pouches. When the fry 

 is ripe the gill is much inflated, and the reflected lamella thus becomes 

 obsolete and looks like the immediate continuation of the convex 

 descending one. Both gills, further, become widely separated, but 

 hold together by means of the interbranchial septum, which thus 

 forms the ventral wall of the single brood-pouch constituted by the 

 two gills. 



The ventral side of the gill, where the filaments bend, is wholly 

 destitute of any marginal furrow, which in other Eulamellibranchia, 

 as well as in all other Pisidia, marks the line of reflection of the 

 filaments. 



On the inside of the gill there are visible the interfilamentar 

 junctions at equal distances ; they are about eight in number. 



The number of filaments in an adult specimen amounts to about 

 twenty-five. In some cases a filament may be confluent with an 

 adjacent one or may branch into two. 



Mantle. — P. clessini shares with P. torquatum the peculiarity of 

 possessing only one siphonal slit, viz. the anal one (PL III, Fig. 1). 

 Here a very short siphon is formed (PI. Ill, Fig. 2), the ventral 

 portion of which is most muscular, whereas the dorsal part has 

 lower and thinner walls. Beside this opening there exists only the 

 long pedal slit. Behind this the mantle coalesces for a rather long 

 space (about twice that of the siphonal opening or the height of the 

 posterior adductor, cf. PI. Ill, Fig. 1). In the mode of fusion, 

 however, an interesting case of variation deserves to be mentioned, 

 which I have observed in specimens from Lake of Luzern (cotypes) 

 as well as in those from Swedish Lappland. Normally, the mantle 

 folds which form the post-pedal suture are entirely coalesced. 

 Occasionally, however, a fissure appears between them which may 

 join the pedal slit so that a prolongation of it arises. In this case 



