31G PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 



sal area are nearly parallel with the longitudinal axis of the animal. 

 The lateral areas are distinct, and the pustules upon them are arranged 

 in rather indistinct lines radiating toward the lateral eiids of the valves, 

 at nearly right angles to the lines on the dorsal area. The sculpture on 

 the mucro is more delicate than elsewhere. The apex of the posterior 

 valve is not sunken, and is not so sharp as in other species compared 

 with it here ; the gkdle is scaly, with also some small spinose transpa- 

 rent scales near the margin. There are five gill-plumes on each side, 

 l^rominent and near the vent. There appear to be two fenestrse on each 

 side. The lateral areas and other portions of the valves are nearly 

 always colored with blackish or ferruginous patches, hut these, as with 

 Trachydermon albus, seem to be really composed of extraneous matter. 



In L.faliginatus Ad. & Rve., the pustules are much smaller, and while 

 having a general longitudinal arrangement on the dorsum, do not form 

 regularly defined rows or chains. The areas are not raised above the 

 dorsum. The shell is much larger and more elevated, with a somewhat 

 sunken and quite sharp posterior mucro. The other mucrones are not 

 raised, but about them the sculpture is more regularly ahgned than 

 elsewhere. I have compared the valves of a typical specimen from 

 Korea collected by Belcher. Eeeve's figure of the sculpture is very bad, 

 as are most of his details. L. alveolus Sars is a very distinct species, 

 though it has been confounded with this. Its sculpture is composed of 

 larger and rather more sparse, isolated pustules, absolutely irregular in 

 distribution and of the same size on the mucro and elsewhere. Ko where 

 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. eoncinnus Gould, from the types, is of a different color, and has a 

 much stronger and dilierent sculpture, like lines of rope. 



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

 but distinguished from either by the peculiar girdle covered with sub- 

 equal scales. 



L. nexus Carj)enter more nearly resembles cancellatus, but the sculp- 

 tui^e is of separate, not lapping, rounded-rhomboidal j)ustules ; the mu- 

 crones 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 

 Middendorf united it, probably without a comparison. 



L. arcticus of Sars seems to be a finely grown variety of this species, 

 if one may judge fi'om the figures ; at least no differential characters are 

 given which seem to be of a j)ermanent character, and not subject to 

 variation within the limits of a species. 



The specimens of this species obtained by me in Alaska were at first 

 referred to fuliginatus by Dr. Carpenter, and some specimens were dis- 

 tributed under that name, or the name of fuliginosus, before I had the 



