158 PROCEEDINGS OF THE MALACOLOGICAL SOCIETY. 



outlined for the purpose of difierentiation. The proximal or 

 pericardial branch of the nephridium begins with a ciliated funnel 

 (cf. Odhner, " Morphologische und phylogenetische Untersuchungen 

 iiber die Nephridien der Lamellibranchien," Zeitschr. f. wiss. 

 Zoologie, Bd. c, 1912, p. 330), describes a short curve towards the 

 front and upwards, and then descends backwards ; in front of the 

 posterior adductor the canal is sharply reflected, and ascends 

 forwards till it debouches into the wide dorsal lobe of the nephridium. 

 That lobe, further, passes into the efferent portion, a wide canal 

 directed medially and, distally, crossing the ciliated tube on its 

 external side, forming here an urinatory sac which debouches to the 

 exterior by means of a pore turned towards the median line of the 

 body (Text-fig. 6, n.p.). 



Seen from the upper side (Text-fig. 3),^the dorsal lobe has, 

 normally, only a slight fissure in its front margin, but this character 

 is not always constant, since exceptionally the fissure may widen 

 to let the tip of the pericardial coil penetrate dorsally (Text-fig. 4). 

 The figures show that an assymmetric structure of the nephridia may 

 occur. 



A comparison with the nephridium of Calyculina lacustris (cf. 

 Odhner, loc. cit., 1912, p. 330, fig. 21) will reveal the fact that 

 P. clessini in its nephridial characters corresponds most closely to 

 an early embryonic stage in the development of the former genus ; 

 it is, moreover, simpler than all the species of Eupisidium. 



Stomach.- — Also from yet a fourth organic system a point of 

 discrimination of P. clessini is to be obtained, viz. the intestinal 

 canal. In all Pisidia the intestine crosses the lower part of the 

 stomach (or the duodenum) on its right side. This is the case in 

 P. clessini, too, but the coil formed by the intestine in front of the 

 duodenum is somewhat longer and describes a simpler course than 

 in other species. Further, the stomach lacks a posterior coecum or 

 pocket, whereas the lateral coecum of the left side is comparatively 

 well developed (cf. Text-fig. 9). 



PiSIDIUM TORQUATUM, Stclfox. 



Gill. — Just as in P. clessini the present species (PI. Ill, Fig. 3) has 

 a single gill on each side, which, however, differs from that of P. 

 clessini in shape and structure ; it has a higher triangular form, it is 

 furnished with a more or less distinct anterior marginal furrow, and 

 it has a reflected lamella extending all along the gill, also the upper 

 filaments thus being reflected (PI. Ill, Fig. 6). The ascending 

 lamella is narrow, occupying only about a third of the breadth 

 of the direct one. The margin of the reflected lamella is free from 

 the body, but a transverse marginal vessel somewhat within its 

 edge connects the gill, with the body wall, and the attachment is 

 continued, further backwards than in P. clessini. Behind the foot 

 the six to eight filaments, which form the posterior lappet of the 



