38 



and undoubtedly there are species, (as in the family 

 Unionidae,) in which the sexes are distinguishable from 

 each" other by certain peculiarities of form and size. This 

 is a portion of the subject which has not yet been investi- 

 gated, and really nothing is known relative to it. There 

 are unquestionably instances in which hybrids occur. 

 These, except under very favorable conditions for observa- 

 tion, would be likely to be regarded as species. 



Occasionally abnormal specimens come to the notice of 

 the Naturalist usually a solitary specimen (and the author 

 " regrets there were not more,") is all that is known, and it 

 is recorded as a species. It sometimes happens that a 

 species produces specimens the epidermis of which exhib- 

 its a uniform color without bands. Other specimens, on 

 the other hand, have bands. Unquestionably, a species 

 has been divided and put on record as two distinct species, 

 with no better ground for the division than has been 

 stated. Again, a species sometimes produces specimens 

 characterized by several revolving elevated lines or carinse. 

 Mr. Say made a second 'species of Goniobasis Virginica, 

 based on this peculiarity ; Mr. Anthony has bestowed sim- 

 ilar attention on a variety of Mr. Say's Anculosa trilineata; 

 and a curious sequel to this is that many intelligent natu- 

 ralists, who do not admit the validity of Mr. Say's " Me- 

 lania multilineata" regard Mr. Anthony's Anculosa costata 

 with favor. 



There are yet other perplexities, and these arise out of 

 the diversities of forms a species is liable to manifest when 

 developed under the varying conditions of stations unlike 

 each other in temperature, mineral properties of the water, 

 abundance or absence of appropriate food, the influence of 

 light, the influence of currents in the water, and finally, 

 other influences of equal importance, which possibly have 

 not yet been conjectured. 



The/orm of a shell is simply a function of the mantle 

 a delicate membrane, in which the soft parts within the 

 shell are included, and which lines the interior of the shell 

 and deposits the calcareous matter of which it is composed. 



