130 Mr. W. Clark on the genus Lepton. 



settle the disputed identity or distinctness between it and L. ni- 

 iidum, and that after due examination I would communicate the 

 result. I redeem my pledge by presenting a short memoir on 

 the genus Lepton. Though the L. squamosum, the type, has been 

 mentioned by authors, I have thought that it would be desirable 

 to give my account of the animal, with some additions and a few 

 observations on the natural position of the genus. 



Lepton, Turton. '^ 



Lepton squamosum, auct. ; 



Solen squamosus, Mont. 



Animal inhabiting a very flat, subrhomboidal, white, porcel- 

 lanous, punctured shell ; its ground colour is a clear white : the 

 mantle is very large, having the margins sinuated, often puckered 

 into two or three folds at the will of the animal ; they extend be- 

 yond the shell more than one-third of the vertical measure at its 

 centre, from which spring a row, on each side the middle of the 

 ventral range, of twenty-five rather long, slender, milk-white ten- 

 tacular pointed filaments ; but the mantle thus clothed is only pro- 

 truded largely beyond the shell, from the middle of the anterior 

 side, throughout the ventral range, to the same level at the pos- 

 terior end j from these points to the umbones it is never seen, 

 being either closed or not protruded, but its suture or edges are 

 furnished with about forty long, strong, blunt, frosted white, 

 rather close-set cirrhi varying in length ; a part of these range at 

 the posterior side of the beaks, above the sessile anal orifice, which 

 occupies a small space without cirrhi, between the termination of 

 the protrusion of the margin and the commencement of the 

 larger filaments on the broader, larger, and posterior side ; of 

 that part of the filaments at the anterior side of the beaks, one 

 is thicker, broader at the base, and double the length of the 

 others; this is the last of the larger ones, which at one time I 

 thought was tubular and might be an oviduct, but further exa- 

 mination seemed to disprove this idea. None of the filaments 

 show much motion ; the long one only, when the animal ad- 

 vanced a step, made an arcuated contraction, similar to that of 

 the fore-finger in extenso when quickly brought down to the 

 palm of the hand ; it then resumed the straight position to await 

 another step : all the other cirrhi are either retractile or con- 

 tractile, separately or en masse. 



The foot is hyaline azure, with a broad longitudinal medial 

 line of intense snow-white, and a still intenser flake at the ante- 

 rior end ; it is fixed to the centre of the body by a moderately 

 long pedicle ; on first protrusion it takes a vertical position, and 



