6 SOLEN1D.E. 



form elongated prisms, with walls as straight and parallel 

 as those of Pinna ; but their axes are nearly conformable 

 with the surface, cropping out somewhat obliquely upon 

 the exterior, where their rounded terminations with 

 distinct nuclear spots may sometimes be seen. The 

 internal layer is very dense, and nearly homogeneous ; 

 but evident traces of cells are occasionally to be met 

 with. Most of the Solen tribe are littoral, and live in 

 sand which they penetrate for that purpose ; a few are 

 found at various and often considerable depths of water, 

 and these prefer a more muddy habitat. None of their 

 remains have been discovered in any geological formation 

 older than the lower tertiaries. 



Genus I. SOLECUR'TUS *, De Blainville. PI. I. f. 1. 



BODY oblong, compressed : mantle capable of being inflated 

 in front : tubes partly separated, extended, and occasionally 

 strangulated, issuing from a common sheath : foot tongue- 

 shaped, of an enormous size. 



SHELL resembling in shape a kidney bean, rather solid, nearly 

 equilateral, concentrically striated or sculptured diagonally 

 with imbricated ribs : teeth, two cardinals in the right valve, 

 and one in the left ; laterals short and rudimentary : pallial 

 scar having a broad and shallow fold. 



For the reasons which I have given in the last volume 

 (pp. 327 and 434), with respect to the systematic value 

 of characters derived from the separation or union of 

 the pallial tubes, I prefer not placing this genus and 

 Ceratisolen in one family, and Solen in another. The 

 relations of Ceratisolen to Solen, through S. pellucidus, 

 are too close to warrant their being assigned to different 

 families, and the transition from the last-named species 

 to S. ensis is very slight and gradual. 



* A short Solen. 



