68 CONCHOLOGT. 



3. Anodonta. Fifteen species. 



A fresh-water shell, found in ponds and lakes, difficult to 

 be distinguished from the Unio but by the hinge, which wants 

 the cardinal and lateral tooth, and merely presents a smooth 

 internal rim round the edge terminated by a sinus or notch, 

 in which the anterior extremity of the ligament is sunk ; the 

 substance is pearly, covered with a false epidermis. 



Shell ordinarily rather thin, regular, close, equivalve, ine- 

 quilateral ; summit anterodorsal; hinge linear, without teeth; 

 ligament external, dorsal, and post-apicial ; two well-marked 

 muscular impressions. 



Anodonta cygnsea. Anodonta rubens. 



A. anatina. A. crispata. 



- A. sulcata. A. uniopsis. 



A. fragilis. A. Pennsylvanica. 



A. cataracta. A. intermedia. 



A. trapezialis. A. glauca. 



A. exotica. A. sinuosa. 



A. Patagonica. 



A. cygncBa. The Swan Anodonta. PI. 11, fig. 2. 



Species oval, thin, elongated, hinge straight, and only au- 

 xiculated anteriorly ; beaks small and ventricose ; concentri- 

 cally wrinkled ; covered with a greenish epidermis, which is 

 frequently of a brown tinge towards the umbo. 



A. ruhens. The ruddy Anodonta, 

 Species oval, hinge arched, without trace of auricula or 

 ear. 



A. trapezialis. The trapezium Anodonta. PI. 11, fig. 3. 

 Species oval or rounded, auriculated on both sides the 

 summit. 



4. Iridina. One species. 



A very rare shell, separated from the Anodonta on account 

 of the hinge being attenuated in the middle, and having small 

 tubercles distributed along its length. The substance is 



