130 APPENDIX. 



and two behind ; the superior one round, that below syphon- 

 shaped, with a large reflexed mouth ; anterior end of the body 

 cleft, and included between the segments of the anterior trans- 

 verse muscle, which is attached to the front of the shell ; abdo- 

 men depressed ; foot conically-elongated, with a tuft of byssus 

 at the base, and a distinct groove for its passage ; posterior half 

 of the gills free throughout. 



Fig. 5, a the byssus, b the lower syphon, c the upper syphon. 

 Fig. 6, the lower syphon magnified. 



ORDER DIMYAIRA. 

 Genus I ANODON. 



Anodon Cygnea, var. piscinalis, pi. XXVII, f. 9, 10. 



Animal with the lobes of the mantle free all round, except 

 at the back ; the posterior margin, when in conjunction, 

 forming two apertures for the passage of the water, food, 

 and rejectamenta, a the upper syphon, which is small and 

 simple, h the lower one, which is provided with a serrated 

 margin, consisting of several series of larger and smaller teeth, 

 c the foot. 



Fig. 10, the lower syphon magnified. 



Genus II.— UNIO. 



Unio pictorum, pi. XXVII, f. 7. 



The animals of the genera Unio and Alasmodon are similar 

 to that of Anodon. a the upper syphon, h the lower one, c the 

 foot. 



Fig. 8 is the spawn of Unio pictorum. 



Genus IV.— CYCLAS. 



Cyclas cornea, pi. XXVII, f. 3, 4. 



Animal with the lobes of the mantle free beneath and ante- 

 riorly, united posteriorly, and extended into two syphons ; foot 

 compressed, and subquadrate, on becoming strap-shaped ; pro- 

 vided with anterior and posterior nearly equal adductor muscles, 

 for opening and closing the valves, a the upper syphon, b the 

 lower one, c the foot. 



