LEPTON. 199 



with an ordinary lens : colour varying from pale yellowish to 

 clear white : epidermis slight, but brilliant and prismatic : 

 margins rounded at each end, much broader at the side 

 furthest from the beak, very slightly curved in front, and 

 somewhat angular at the back : beaks prominent and projecting 

 beyond the dorsal margin, placed nearly in the middle, but a 

 little towards the narrower end of the shell : hinge-line curved, 

 occupying about one-fourth of the circumference: cartilage 

 small and slight, light horncolour, placed close to the beaks on 

 the narrower or posterior side : hinge-plate narrow, incurved 

 or excavated in the middle : teeth as in L. squamosum, but they 

 are more developed and lie more compactly, the cardinals are 

 more oblique, and the laterals are smaller, and placed nearer 

 the hinge-line in consequence of the groove which contains 

 these teeth being narrower and deeper in proportion to the 

 size of the shell : inside highly iridescent, and microscopically 

 striated lengthwise; margin plain: scars distinct. L. O'l. 

 B. 0-125. 



Var. 1. convexa. Shell rather more convex, and coarsely 

 pit-marked throughout. L. convexum, Alder, Cat. Moll. North. 

 <fc Durh. p. 93 ; F. & H. ii. p. 102, pi. xxxvi. f. 10. 



Yar. 2. Uneolata. Shell rather natter than usual, and marked 

 with minute longitudinal lines. 



Var. 3. pisidialis. Shell triangular and convex, resembling 

 in shape Pisidium pusillum (var. obtusalis), marked with ex- 

 ceedingly minute wavy ribs, which are formed by the con- 

 fluence of the pit-marks. 



HABITAT : Gravelly sand on most parts of our coast 

 from the Shetland to the Channel Isles, in from 10 to 

 90 fathoms : although local, it is not uncommon. It is 

 a Coralline Crag fossil. Var. 1. Equally diffused in a 

 recent state. Yar. 2. Loch Fyne (M'Nab) ; Exmouth 

 (Clark) ; Guernsey (J. G. J.). Var. 3. Torbay (Web- 

 ster). The typical form lias been found by Malm on the 

 Swedish coast in 20-50 fathoms, and by myself in the 

 Gulf of Spezzia in 18 fathoms. Sars has noticed its oc- 

 currence in postglacial beds in the Christiania district. 



According to Mr. Clark the anterior part of the animal 



