124 CLASS I. TROPIOPODA. ORDER I. BIMUSCULOSA. 



in the shell to that of many of the Naiades, we leave it under the im- 

 pression that a future investigation of the animal will serve to establish its 

 claim to a new and and separate family. Gray has indeed arranged the 

 Mycetopi in a particular family, Mycetopodida;, but we anticipate a higher 

 rank for these animals than that which is assigned to families in his 

 method of classification ; we cannot think that the propriety of distin- 

 guishing his Unionida; and Iridinidte as separate families, is equal to that 

 of distinguishing his Unionidce and MycetopodidcB. The former are inti- 

 mately allied by the Hyria and Anodontes ; but the latter are apart, and 

 without their intermediate varieties*. 



The shell of Mycetopus may be described as being thin, transverse, 

 equivalve, nearly equilateral, and of a long cylindrical form, gaping at 

 both ends, more particularly at the anterior. It is covered with a dark 

 olivaceous epidermis, somewhat rubbed off towards the umbones, and 

 the interior is lined with a slight nacre ; the umbones are central, and 

 turned directly inwards to each other. The hinge is linear, straight, 

 and without teeth, furnished with a thin marginal ligament. The mus- 



* In speaking of Gray's classification, it must not be supposed that we can fail to estimate 

 his very skilful distribution of the Mollusca ; as far as we are enabled to judge by his neces- 

 sarily brief sketch of it in the ' Museum Synopsis,' it appears to be based upon a careful ob- 

 servation of the animal. We only object to his great extension of the nomenclature : we 

 admire his arrangement, while we regret the introduction of such a confusing multiplicity of 

 generic names ; conceiving that it would far more promote the interest of science, if the ela- 

 borate detail of his classification were carried out in the sectional subdivision of well-known 

 genera, — in a language with which the world is familiar. The love of science is endangered 

 when men of credit and ability arrogate to themselves the right of disturbing the received no- 

 menclature ; no new theories should be started, unless called for by the appearance of new 

 phenomena. All this, however, might be allowed, if his system of arrangement were fairly illus- 

 trated by the great national collection of shells under his control ; we might then appreciate 

 his alterations, and be the very first to acknowledge the reasonableness and propriety of what 

 now seems to come forth in the spirit of innovation. In the description and appropriation 

 of what we call species, it is different ; here every man may use his own fancy and discretion : 

 his ingenuity of comparison is stretched to the utmost, because he cannot keep pace with the 

 creative force that is in operation ; new forms are constantly making their appearance, because 

 there is no limit to the creation of kinds ; and what is a species ? the operations of Nature are 

 infinite, — she does not choose to be defined. 



