LEPIDOPLEURUS. 5 



isolated pustules, absolutely irregular in distribution and of the 

 same size on the mucro and elsewhere. Nowhere do they form 

 lines. The arch of the back is peculiarly round, the lateral areas 

 not raised and barely distinguishable. The girdle seems similar. 

 I have compared typical examples. 



L. concinnus Gould, from the types, is of a different color, and 

 has a much stronger and different sculpture like lines of rope. 



L. internexus Carpenter and var. rugatus Cpr., are more like con- 

 cinnus, but distinguished from either by the peculiar girdle covered 

 with subequal scales. 



L. nexus Carpenter more nearly resembles cancellatus but the 

 sculpture is of separate, not lapping, rounded rhomboidal pustules ; 

 the mucrones are much more pronounced, and the white ground is 

 prettily marbled with black and gray inherent coloration. The 

 name cancellatus is a misnomer, since it is only in certain lights that 

 any trace of reticulation can be observed faintly. The young are 

 flatter than the adults. It bears no resemblance to L. asellus, with 

 which Middendorff united it, probably without a comparison. 



L. arcticus of Sars seems to be a finely grown variety of this spe- 

 cies, if one may judge from the figures; at least, no differential 

 characters are given which seem to be of a permanent character, 

 and not subject to variation within the limits of a species. 



The gills occupy a space corresponding to the posterior quarter 

 of the foot ; there are about eight or ten on each side. The 

 mantle edge is plain and thick. The veil is plain. The muzzle is 

 rounded, with a little papilla at the posterior corner on each side 

 (Dall.) 



L. ARCTICUS Sars. PI. 3, figs. 59-63. 



Shell subdepressed, dorsal keel obsolete, lateral areas little con- 

 spicuous. Posterior valve smaller than the anterior, subrhom- 

 boidal, roundly produced in front. Median plates three times as 

 long as broad, the front margin truncate in the middle. Surface of 

 the valves nearly smooth, indistinctly granulose. (Sars.) 



Length 12 mill. 



Spitzbergen, Greenland, Vadso and Finmarlc> 20-100 fms. 



Lepidopleurus arcticus SARS, Moll. Reg. Arct. Norv. p. 112, t. 7, 

 f. 7. Chiton arcticus JEFFREYS, P. Z. S. 1882, p. 668. 



Dall, judging from the description and figures, considered this a 

 form of L. cancellatus. Jeffreys, who found the species in the 



