FAMILY NAIADES. 211 



Cardiacea. Animal bearing a thin horny shell, with the 

 mantle lobes open anteriorly for the passage of a large pro- 

 truded foot, and united posteriorly to form the branchial and 

 excretory siphons which are prolonged into tubes wholly or 

 partially united. 



Family I. NAIADES. 



Animal bearing a pearly shell, with the mantle lobes freely open except 

 behind, where they are united to form the branchial and excretory 

 siphonal orifices, which are simply pouted. Foot large, free. 



The Acephalous mollusks are less numerous in kind than the 

 Cephals, but they include species of much larger dimensions. 

 Among those inhabiting the sea, the giant clam (Tridacna) is of 

 greatly more colossal proportions than any shell of spiral growth ; 

 and in freshwater, the Paludina, largest of our pond-snails, is far 

 exceeded in size by the Naiad. The lakes and rivers of both 

 hemispheres, especially the western, teem with this mollusk, a 

 pearly bivalve of mud-dwelling habit, with a large tongue-shaped 

 delving foot. The mantle lobes are freely open, excepting behind, 

 where they are united to form the branchial and excretory siphonal 

 orifices ; the edges round the branchial orifice being sharply fringed, 

 and the hinge margin of the shell is sometimes toothed, sometimes 

 without teeth. 



Such are the typical characters of the family, but there are four 

 very distinct and remarkably local foreign genera included under 

 this head, in which the structure of the foot and posterior siphons 

 is greatly modified. Two, Iridina and Spatha, inhabitants of the 

 Nile and rivers of Senegal, have the posterior siphonal orifices pro- 

 longed into short unequal tubes. The other two, Hyria and Myce- 

 tojous, natives of South America, are more different from each other. 

 In Syria, the siphonal orifices are prolonged into tubes. In Myce- 

 topus there are no siphonal tubes, but the foot is singularly long 

 and attenuated, reaching a considerable extent beyond the shell, 

 and at the extremity it is suddenly widened into a knob. Myceto- 

 pus has a curiously long and narrow shell, gaping at both ends. 



The shell of the Naiades is of an iridescent pearly substance, 

 covered by a dark fulvous -olive, green, brown or black epidermis, 

 and all more or less produce pearls. One or two of the North 



p 2 



