SPECIES OF 



Pfeiffer's diagnosis of the genus Pisidivm is as follows (134, 

 Abt. i, pp. 17 & 123) : 



* ; PISIDIUM, Nobis. 



" Thier : statt der rohrenformigen Tracheen ein schmaler 

 fleischiger Vorstoss an dem vorderen, Theile der Schaien ; 

 der Fuss lang und schmal. 



" Gehiius : langlich, ungleiehseitig, voilig schliessend ; an dor 

 rechten Schale ein, an der liuken Schale zwei gegeneinander 

 iiberstehende, sehr kleine Hauptzahne ; nach hinten und 

 vorne zwei diinne, lamelleiiformige Seitenziihne; letztere an 

 der rechten Schale gespalten, um die gegen iiberstehende n 

 auf/.unehmcn." 



The shell -characters may be more narrowly defined as follows : 

 Shell close, ovoid to orbicular, equivalve, inequilateral, posterior 

 side the shorter, more or less concentrically striate, thin, with 

 olive-horny periostracum, sometimes, especially in young individuals 

 and certain species, sparsely covered with short hairs ; umbones 

 sometimes prominent, sometimes tumidly obtuse, nepionic shell 

 usually smooth ; ligament subexternal ; hinge formula* : 



R.V, a. i : a. in 

 L.V. : a. n : 



R.V. 



L.V. 



Fig. 1. Topography of the hinge of Pisidium: B.V. Right valve; L.V. Left 

 valve ; a.i-ni, anterior lateral teeth ; c.2~4, cardinal teeth ; I, ligament ; 

 p.i-iij, posterior lateral teeth. 



* Felix Bernard's nomenclature for the hinge (14) has been followed as 

 being the most scientific. R.V. = Right Valve ; L.V. = Left Valve; a=anterior 

 lateral teeth; <?=cardinal teeth ; /ligament ; and p= posterior lateral teeth. 



Bernard points out that the hinge in the Eulamellibranchia arises in the 

 form of folds parallel with the shell-margin. With growth, deposits of shelly 

 matter at certain points eventuate in the formation of the teeth of the adult 

 shell. In the group to which Pisidium belongs, in the right valve c.3 is at 

 first continuous with a. in, and in the left valve, c. 2 with a. //, c. 4 with a subse- 

 quently suppressed a.iv. In the species of Pisidium subsequently described 

 in this Catalogue, it will be noted that all except P. amnicum and P. astartoide* 

 show traces of the former continuity between c.2 and a.n. 



