14 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.54. 



tance within the margin, but inside of these only a few scattered 

 bristles are evident, and taken as a whole the girdle is remarkably 

 free of them. Most of them are quite broken away in my material, 

 but a series of the stumps or ''pores" can be made out running clear 

 around the girdle, most of them corresponding to the sutures in 

 position, though there are at least 6 or 7 behind the tail valve, and 

 2 in the same series on either side of the head valve. The extreme 

 margin of the anterior lobe is decorated with a single series of short, 

 very spinose bristles, between and beneath which occur a row of 

 spines springing directly from the substance of the girdle itself (pi. 

 8, fig. 5). Toward the front of the lobe the spinose bristles are 

 quite close together, but, although continued around the entire lobe, 

 they become progressively smaller and more infrequent toward the 

 sides and rear. The anterior margin frequently, if not always, ex- 

 hibits a microscopic crenulation corresponding roughly to the bristles 

 so that from certain aspects the latter appear as though borne upon 

 small lobes. The contrast between the two types of bristles (i. e., 

 marginal and dorsal) is very marked. The marginal bristles (pi. 10, 

 fig. 6) have a bushy appearance, due to the very numerous spines, 

 which, though not actually curved, often appear so because of the 

 angle at which they project from the more or less twisted core. The 

 dorsal bristles (pi. 10, fig. 4) are very narrow, trim, and slender, but 

 larger, their long straight spinelets being much less crowded and 

 closely apphed to the core of the bristle for practically their entire 

 length. Their arrangement is throughout very neat and regular. 

 A cross section shows only about 8 spines to the tier on this type of 

 bristle. The bristles do not appear to be banded, but are nearly 

 uniform in color. 



The radula (pi. 10, fig. 9) is of the same type as that of P. stimpsoni 

 and P. velata, but the teeth show numerous differences in detail. 



The ctenidia number 22-24 on each side. 



The species has not been reported since its foundation by Pilsbrj 

 upon the valves of a single specimen obtained by Grebnitzki at 

 Bering Island. The present record, therefore, constitutes a very 

 appreciable extension of the known range. Despite his inadequate 

 material, Pilsbry succeeded in acutely discriminating all the more 

 important shell characters. P. borealis is, in fact, a very distinct 

 species, and in the radiate sculpture of the anterior valve, complete 

 sinus, and peculiar bristles, possesses features sufficient effectually to 

 prevent confusion with any of the other described species, unless we 

 consider the Placophoropsis group, the members of which are well 

 separated by their subgeneric characters. 



From P. velata and P. stimpsoni the more sparsely hairy girdle 

 and much weaker armature of the bristles are striking differences. 



