FAMILY 12. NAIADES. 115 



was originally selected by Lea as a type for the formation of a new genus, 

 Symphynota ; upon finding, however, that the character was not only 

 irregular, but more or less common to many of the Naiades, he aban- 

 doned it. Even as a general rule of division, we have but little confidence 

 in the connate character, as it may be found to vary in the same species 

 at different periods of growth. 



In consequence of the gradual and perfect link which holds this family 

 in connexion, we have decided upon a still more simple distribution, 

 relying upon the typical characters of five genera only, as best calculated 

 to simplify the arrangement. How truly we see that there is really no 

 arbitrary division of species in nature, but that the limits which are 

 devised by the ingenuity of man for the arrangement of created things 

 are wholly artificial, and become sooner or later destroyed by the con- 

 stant multiplication of intermediate forms and varieties ! 



The animal of the Naiades appears to be nearly the same throughout, 

 excepting in the genus Iridlna, where it is said to be armed with two 

 tubes or siphons, formed by the prolonged union of the hinder parts of 

 the mantle. Their shell may be described as being inequilateral, having 

 the valves connate or free, covered with a thick olivaceous epidermis, 

 and more or less eroded at the umbones. The hinge is very irregular ; 

 there are generally two or more very thick solid teeth, often none ; 

 sometimes the hinge-margin is furnished with an irregular series of 

 tubercles, but these also are often obsolete. The valves are united by a 

 strong ligament, and the anterior muscular impression is always com- 

 pound, showing the marks of one or more accessory cartilages of attach- 

 ment. 



The Naiades are fluviatile, and are found abundantly upon the banks 

 of rivers, after being washed up by the tide. They are divided into 

 five genera, as follows : 



Q '1 



