FAMILY 12. NAIADES. 123 



MYCETOPUS, D'Orbigny. 



Testa tenuis, transversa, elongatissimo-cylindrica, subarcuata, a?quivalvis, 

 subrequilateralis, utrinque bians prrecipue ad extremitatem anticam, 

 epidermide brunneo-viridescenti induta, intus cserulescens ; umbo- 

 nibus centralibus, biangulosis, ad ipsos incurvis. Cardo linearis, 

 rectus, edentulus ; ligamento tenui, raarginali. Impressiones mus- 

 culares subindistinctse, composita?. 



The genus Mycetopus was introduced by D'Orbigny for the purpose of 

 characterizing a new fluviatile mollusk inhabiting the rivers of South 

 America. It was first discovered by him at Santa Cruz in the Republic 

 of Bolivia ; it is described in his ' Synopsis terrestrium et fluviatilium 

 Molluscorum in suo per Americana Meridionalem itinere,' and figured in 

 his ' Voyage dans l'Amerique Meridionale.' The Mycetopi have little 

 affinity with the Iridince : the lobes of the mantle are free, and there are 

 no siphons ; their great peculiarity is in having a long extended cylin- 

 drical foot, inflated at the extremity, which they have not the power of 

 withdrawing. By the assistance of this foot they are said to perforate 

 like the Pholades ; we cannot, however, venture upon this assertion until 

 their characters and habits are satisfactorily known. We only place the 

 Mycetopi in this family provisionally, because, after an accurate observa- 

 tion of the shell from which our figures are drawn, we are strongly 

 inclined to think that they differ essentially from any other of the 

 Naiades. Besides the peculiarity of the foot, there is a general tenor of 

 character in the shell, especially in gaping widely at both ends, that 

 seems to indicate a new organization in its animal inhabitant. The 

 soleniform elongation of the valves, their tenuity, and the very central 

 position of the umbones, are all minute evidences of a change of character. 

 For the present, however, we can but generalize upon the slender informa- 

 tion we are in possession of; for although there is a certain resemblance 



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