FAMILY MYTILACEA. 



207 



Our freshwater representative of this family is referred to a 



special genus : — 



1. Dreissena. Animal bearing a triangular fan- shaped shell, 



mantle closed throughout, except for the passage of the foot 



and of two tubular orifices for the purposes of excretion and 



respiration. 



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Dreissena polymorpha. 



Genus I. DREISSENA, Van Beneclen. 



Animal; subtriangular, rather compressed, bifurcated in front, 

 bearing a subtriangular shell of which the umboes cover the 

 bifurcations, mantle delicately fringed, closed throughout, ex- 

 cepting a small opening in front for the passage of the foot and 

 two tubular orifices behind for excretion and respiration, foot 

 ligulate spinning a copious byssus, excretory orifice developed 

 into a small conoidal siphon, respiratory orifice larger, subtu- 

 bular, armed with longitudinal rows of scale-like papilla?, of 

 which those on the inner margin are elongated internally to- 

 wards the centre. 



Shell ; equivalve, very inequilateral, triangular, very gibbous, um- 

 boes terminal, furnished in each valve with a cross septum for 

 the adhesion of a small adductor muscle, cicatrix of the prin- 

 cipal adductor muscle oblong, compound, situated within the 



