292 Mr Jenyns's Monograph on the 



on the land and fresh water Mollusca of Germany*, first insti- 

 tuted the genus Pisidium for the reception of those species which 

 are characterized by an inaequilateral shell, and a single siphon 

 at the posterior extremity of the cloak. This distinction, founded 

 upon the structure of the animal as well as that of the shell, 

 appears to be perfectly natural ; while there is little doubt that 

 its utility will be soon acknowledged, when increased attention 

 shall have been paid to this family, and future discovery have 

 still further augmented the number of species, which we have 

 every reason to expect will be the case. The propriety, however, 

 of such a division of the old genus Cyelas will be best seen from 

 the following comparative view of the characters of those adopted 

 in this paper. 



Gen. I. CYCLAS, Lam. Pfeiff. 



Animal: Pallium antice, pro pede exserendo, apertum, postice con- 

 natum, et in tubum siphonalem longum, duplicem, contractilem, exten- 

 sum. Pes linguiformis, valde extensilis. 



Testa corticata, suborbicularis, suba?quilateralis. Dentes cardinales 

 minuti ; in dextra valvula unicus plus minusve complex ; in sinistra 

 duo oblique collocati. Dentes laterales longitudinales, compressi, lamel- 

 liformes, in valvula dextra subduplicati. Ligamentum externum, posti- 

 cum, lateri longiori insitum. 



Gen. II. PISIDIUM, Pfeiff. 



Animal: PalUum antice, pro pede exserendo, apertum, postice con- 

 natum, et in tubum siphonalem brevem, simplicem, contractilem, ex- 

 tensum. Pes linguiformis, valde extensilis. 



* Systematiscke Anordung und Beschreibung deutscher Land-und Wasser-Schnecken, §c. von 

 Carl Pfeiffer. Cassel, 1821. 



