316 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 pnstules npon them are arranged 

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

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

 the mncro is more delicate than elsewhere. The ajiex of the posterior 

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

 with it here ; the gh'dle is scaly, with also some small spinose transpa- 

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

 prominent and near the vent. There appear to be two fenestra on eacli 

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

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

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



In L.faliginatus Ad. & Eve., the x>ustules are much smaller, and wiiile 

 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 sharj) j)osterior mucro. The other mucrones are not 

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

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

 Korea collected by Belcher. Eeeve's figure of the sculi^ture 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 sculptiu-e is composed of 

 larger and rather more sx>arse, 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, fi-om the types, is of a different color, and has a 

 nruch stronger and different sculpture, like lines of rope. 



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

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

 equal scales. 



i. nexus Carpenter more nearly resembles cancellatus, but the sculp- 

 ture is of separate, not lapping, rounded-rhomboidal imstules; 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 aie 

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

 Middendorf united it, probably without a comparison. 



X. arctmis of Sars seems to be a finely grown variety of this species, 

 if one may judge from the figures ; at least no difierential characters are 

 given which seem to be of a permanent character, and not subject t() 

 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 fuUginosiis, before I had the 



