155 



ON THE ANATOMICAL CHARACTERISTICS OF SOME BRITISH 



PISIDIA. 



By Nils Hj. Odhner. 



(Commumcated by B. B. Woodward, F.L.S.) 



Bead 10th November, 1922. 

 PLATE III. 



In a previous paper (" On some species of Pisidium in the Swedish 

 State Museum," Journ. of Conch., xvi, 1921, pp. 218-23) I drew 

 attention to the anatomy of Pisidium, and pointed out that this 

 gives important points for the taxonomy in that genus hitherto 

 analysed by the systematists chiefly on conchological grounds. 



The most useful characters in question are to be found in the 

 number and structure of the gills, in the mode of fusion of the mantle 

 margins, and in the shape of the nephridia. In the present article 

 I give some exemplifications and illustrations of these particulars, 

 choosing for subject two of the British species belonging to the sub- 

 genus Neopisidium (with one single gill on each side and a single 

 siphonal slit), and a third species representing Ewpisidium. The 

 species in question are P. clessini, Surbeck, P. torquatum, Stelfox, 

 and P. personatum, Malm (cf. Odhner, loc. cit.). Some details are 

 given of other species for comparison ; but a more thorough report 

 on those will be published at another occasion. 



The British material forming the base of this investigation was 

 kindly sent to me by Mr. Oldham ; in addition, specimens from other 

 countries have been used for completion of the details. 



In the first place I will remark that the conditions described later 

 can only be well observed under considerable magnification (about 

 thirty to sixty times), and consequently a very sharp oblique light 

 must be used. 



Pisidium clessini, Surbeck. 



Gill. — This species (PL III, Figs. 1, 2) is the most simply organized 

 of all British Pisidia. There is only one gill (or demibranch) on each 

 side, representing the inner one in most of the Lamellibranchia, 

 or the anterior one in Eupisidium. The gill (PI. Ill, Fig. 5) consists 

 of the direct lamella only ; but a feeble trace of a reflected lamella 

 may be considered to be present. This is, however, very incomplete, 

 and restricted to the middle and lower parts of the gill. The upper- 

 most filament is, jUst as in all Pisidia, coalesced all along the body 

 side. The upper (about eleven) filaments have a direct course from 

 the gill axis (a) towards the front, where they end without bending. 

 Beneath this portion of the gill its filaments have reflected ends ; 

 the whole of them taken together thus forms an indistinct reflected 

 lamella. Only the uppermost six filaments coalesce with the body 

 surface at their ends ; all the remaining ones have their ends free 



