220 JOURNAL OK COXCHOI.OGY, VOL. l6, NO. 7, SEPTEMBER, tg2t. 



into the calcareous layer of the shell but not into its cuticle ; in these 

 canals are contained fine threads from the pallial epithelium, which 

 is thus fixed to the inside of the valves. The nature of these canals 

 is most evident in the small and fragile P. c/essi'ni, where they run 

 not perpendicular to, but parallel with, the shell surface, probably as 

 a consequence of the thinness of the shell ; they are, moreover, easily 

 seen from the outside. Often there may be seen two or more canals 

 branching from a common pore, a character peculiar to P. c/essini. 

 The contents of these canals were easy to demonstrate by infusing 

 some decalcifying liquid. When the shell was dissolved — a process 

 which could be directly followed under the microscope — there 

 remained the thread-like contents of the vanished canal, issuing from 

 an epithelial cell of the mantle. 



This porous structure of the Pisidium shell has also been detected 

 independently and almost simultaneously by Mr. Stelfox. 



Below I give some notes on a few species which have been subject 

 to much discussion or else are unsatisfactorily known. I beg to 

 express my thanks to Mr. Stelfox, who has given me much informa- 

 tion in discussing the matter with me. 



Pisidium torquatum Stelfox, 1918. 

 (/*. parviihnii Woodward, 1913, non Clessin, 1873). 



Four specimens of this species are present in the Stockholm 

 Museum, labelled " P. conicum Baudon, Dinkelscherben, S. Clessin," 

 and the label is written in Clessin's characteristic hand. The largest 

 specimen measures 2 mm. in length, and i"6 mm. in height. The 

 German examples differ somewhat in shape from those found in the 

 Fure Lake in Denmark, being more lengthened and having the angle 

 on the anterior slope less accentuated. The umbonal plaits are, 

 however, typical, and the striation is irregular. Woodward gives as a 

 synonym for the present species P. aliemim Martens, but I have failed 

 to find any description of such a form. There exists certainly a 

 Pisidmiii alienum of Clessin (Martini and Chemnitz, Conch. Cab., 

 1877), but this is a large and quite different shell. Under these cir- 

 cumstances the species should be named torquatmn, as proposed by 

 Stelfox (19 1 8), 



I have had the opportunity to examine its anatomy in Danish 

 specimens kindly sent me by Dr. Steenberg, and it is of great interest 

 to be able to state that the soft parts differ from those of our common 

 species of Pisidium, inasmuch as there is only one gill on each side 

 (in other Pisidia generally two, of which the posterior is less deve- 

 loped) ; moreover, there is only one siphonal opening, viz., the anal 

 one, the branchial being contained in the pedal slit ; further, the 

 nephridium is shaped somewhat differently, and shows, seen from the 



