80 ARCADE. 



equally important is, that the side anterior to the beaks is 

 double the transverse length of the posterior one ; whereas in 

 all the other Leptons, of which I have more than a hundred 

 examples, the beaks are nearly central, and they have more or 

 less subangularity at the sides ; therefore the oblique outline, 

 rounded sides, and position of the beaks, are unerring guides 

 to distinguish the L. Clarkits from its congeners. 

 The animal has not been observed. 



L. CONVEXUM, Alder. 



L. convexum, Brit. Moll. ii. p. 102, and iv. p. 255, pi. 36. f. 10, mag- 

 nified. 



L. nitidum, Turton et Auct. 

 Kellia nitida, Brit. Moll. ii. p. 92, pi. 36. f. 3, 4. 



The animal inhabits a light greenish-yellow or pure white, 

 subrhomboidal, moderately convex, more or less punctured 

 shell. The mantle is frosted- white with the margins plain, 

 but as much proportionately protruded beyond the edge of the 

 shell as in L. squamosum ; it is in like manner clothed with 

 cirrhal filaments of about the same length, and of pruinose 

 white, but unlike that species, they are rather less developed 

 dorsally than ventrally ; each filament at its terminal edge is 

 studded with four or five white points or cilia, so sharp and 

 minute as to require a powerful lens to see them. There is no 

 conspicuous leading process, as in the preceding species, but 

 the mantle, at the same anterior point, forms a visible pro- 

 jection or fold. In this species, and contrary to L. squamosum, 

 the longer and broader end is anterior, but the beaks are so 

 central, that there is little difference in the sides ; the single 

 sessile anal tube is exactly as in the last species ; there is no 

 branchial siphon, the water enters at the extensive ventral 

 aperture. The foot is almost in every respect similar to that 

 of its congener ; it is perhaps larger in proportion, of pale 

 azure hue, marked with intense but irregular flake-white 

 minute blotches; the posterior extremity is as long as the 

 portion anterior to the pedicle ; its termination is perfectly 

 aciculate, and like its congener deeply grooved as far as the 

 junction with the body, at which point is the byssal gland, 



