SPECIES OF rrsimr\r. 



Pfeiffer's diagnosis of the genus Ptsidivm 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 Schalen ; 

 der Fuss lang und schmal. 



" Gehaus : liinglich, ungleichseitig, voilig schliessend ; an der 

 rechten Schale ein, an derlinken Schale zwei gegeneinander 

 iiberstehende, sehr kleine Hanptzahne ; nach hinten und 

 vorne zwei diinne, lamellenforruige Seitenziihne; letztero an 

 der rechteu Schule gespalten, urn die gegeniiberstehenden 

 auf/.unehmcn." 



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

 Shell close, ovoid to orbicular, eq nival ve, inequilateral, posterior 

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

 olive-horny periost.racum, 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./ : a, HI 

 L.V. :.//: 



R.V. 



L.V. 



cutt 



Fig. 1. Topography of the hinge of Pis idium : R.V. Right valve; L.V. Left 

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

 p.i-in, 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 ; c=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 <?..? is at 

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

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

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

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



