DREISSENID^. 19 



soft parts excepting as regards the general shape, 

 which corresponds to that of the shell ; but I have 

 observed in A. anatina, that the branchial opejiing is 

 not only comparatively but actually much -larger, 

 and fringed with much more delicate and numerous 

 tentacles than in Anodonta cygnear In any case, 

 whether a separate species or a variety only, it may 

 be distinguished from A. cygnea by its shell being 

 smaller, and longer in proportion ; by the hinge line 

 being raised instead of straight, and by the abrupt 

 instead of gradual slope of the posterior side. 



Var. I. radiata.—'^'^^ marked with green and yellow rays. 

 " The rayed markings form scarcely a varietal character, being 

 common to half-grown individuals of the last, as well as of this 

 species." — ^.C,vol. i. p. 44. 



Var. 2. ventricosa. — Shell larger, more solid, exceedingly 

 tumid, especially in the middle and towards the umbonal region, 

 also marked with green and yellow rays. River Exe,^.C Near 

 Birmingham (G. Sherriff Tye), J.C. Near Colchester (Laver). 



Var. 3. complaiiata. — Shell oval, greatly compressed, brown ; 

 beaks placed close to the anterior margin ; upper margin raised 

 and curved ; anterior side abruptly truncate. Gumfrieston, 

 near Tenby, B.C. Brook near Harpsden Wood, Henley-on- 

 Thames (Rich). 



FAMILY III.— DREISSENID-ffi. 



Body rhomboidal, compressed ; mantle closed, with the ex- 

 ception of three orifices, two of which are placed on the posterior 

 side ; the upper one is short and serves as the excretal passage, 

 the lower one which is produced (lengthened) is the respiratory 

 opening, and its margin is reflected, and fringed with spine- 

 shaped tentacles : the third orifice is placed on the front margin 

 and serves for the passage of the foot ; foot tongue-shaped, with 

 a byssal groove. 



Shell triangular, equivalve, inequilateral ; epidermis strong, 

 horny ; beaks situated at the anterior end ; ligament internal ; 



C 2 



