160 PBOCEEDINGS OF THE MALACOLOGICAL SOCIETY. 



Along the margin, beneath the line of attachment, the marginal 

 vessel runs, transporting the venous blood from the cephalic region 

 to the filaments. This combination of the marginal vessel with the 

 edge of the inner lamella is a characteristic met with in all Eupisidia 

 in contradistinction to Neopisidium, where the- vessel has been 

 disconnected from the edge and fused to the inside of the direct 

 lamella. 



Towards this margin the filaments of the reflected lamella are 

 free from the opposite branches in the direct lamella, but in the lower 

 two-thirds of their length the respective branches are intimately 

 fused (PI. Ill, Fig. 7a), the reflected lamella thus becoming fused 

 with the direct one for two-thirds of its breadth. This interlamellar 

 fusion is a simple coalescence without any development of septa 

 between the branches, a higher development that occurs in other 

 species, e.g. P. casertanum and henslowanum ; in the latter, further, 

 the septa alternate in height. In the present form the connexion 

 is extended to an equal height in adjacent filaments, though 

 decreasing in height posteriorly. , 



In the rear portion of the gill the filaments are elevated on their 

 inside, thus forming the pockets usual in fertile specimens. 



There are about thirty-five filaments in the foremost gill and about 

 eight interfilamentar junctions. The foremost crest of the gill is 

 furnished with a distinct longitudinal furrow marking the line of 

 reflection. 



Compared with the anterior gill, the posterior one is rather small 

 in size (PL III, Figs. 4, 7) ; it forms a narrow stripe behind the gill 

 axis and occupies its lower half only. In other species of Pisidium 

 the posterior gill generally attains a greater height. This gill is 

 homologous with the posterior (or external) one in Sphcerium and 

 other Eulamellibranchia, and consists entirely of the reflected 

 lamella, the descending one being completely obsolete and repre- 

 sented only by the axial portions of the filaments. An indication 

 of a direct lamella may be found occasionally, e.g. in P. amnicum. 



Mantle. — On examining the mantle we find the pedal slit to be 

 extended far backwards ; the length of the post-pedal suture 

 equals the height of the posterior adductor (and the siphonal 

 opening), and surpasses considerably the diameter of the branchial 

 slit. Further, it is to be mentioned that the inner mantle fold is 

 somewhat thickened but essentially of about a uniform breadth 

 throughout its length, this being in sharp contrast to P. suhtruncatum, 

 in which the pedal slit is much smaller and the post-pedal suture 

 correspondingly very elongated ; the mantle fold, further, of that 

 species is much more swollen in its foremost portion than posteriorly. 



The shape of the siphon is described and figured by Phillips and 

 Stelfox (1918). 



Nephridium. — The nephridium of P. personatum (Text-fig. 7) 

 forms a step towards a higher development than that of P. clessini. 



