89(5 UNITED STATES GEOLOGICAL SUKVEY OK THE TERRITORIES. 



as in the details of its septa; some specimens tapering more rapidly than 

 others, so that measurements of the angle of divergence from the smaller 

 extremity are not always precisely the same. In most of the medium-sized 

 and smaller fragments, the lateral ridges, or undulations, are nearly or quite 

 wanting, as in that figured on our plate 20*; while, in other larger pieces of 

 the non-septate portion, they are often well marked, as in that illustrated in 

 the Memoirs of the American Academy of Arts and Sciences, Boston, V 

 (n. s.), plate v, fig. 1 a. 



Some of the more strongly undulated specimens of this shell resemble 

 ' B. anceps, Lamarck; but it is evidently a much larger, more robust, and more 

 rapidly tapering species, and is never carinated on the siphonal side, as B. 

 anceps is illustrated by d'Orbigny and some other European authors; while 

 the lobes and sinuses of its septa are proportionally deeper, and differ some- 

 what in their details. In addition to these differences, none of our numerous 

 specimens, of such small sizes as those of B. anceps often figured with strong 

 lateral undulations, show this character. 



It seems, however, to be more nearly allied to B. Faujasi, Lamarck, 

 which some confound with B. anceps. In its oval section, rounded siphonal 

 side, and even the details of its septa, it certainly agrees very nearly with B. 

 Faujasi, as sometimes illustrated by European authors. But its very much 

 larger size, more robust general appearance, and more rapidly tapering 

 form, seem to me to separate it entirely from B. Faujasi. Dr. Binkhorst, 

 however, cites it as a synonym of that species in his Monograph of the Gas- 

 teropoda and Cephalopoda of the Upper Chalk of Limbourg; but for the 

 reasons already mentioned, I cannot agree with him on this point. 



Of the variations of form, etc., noticed among tin 1 specimens believed to 

 belong to this species, the following are worthy of note: first, those with a 

 very regularly ovate section, such as represented by our figures 2, a, b, plate 

 20, which seems to be the most common form among the Upper Missouri 

 specimens. Occasionally, however, we meet with individuals presenting a 

 more broadly ovate section, as shown by our figure 1, l\ of plate 20; and 

 although slight differences may be observed in the details of the septa ol 

 some of the specimens of these two forms, as shown in our figures 1, a, and 

 2, d, of plate 20, these differences are not greater than may be observed 



"The outline-restoration of the aperture in this figure, as well as that of B. conipressus on the same 

 plate, is intended to illustrate the form of the lip only, and not the length of the non-septate last cham- 

 ber, which is niueh longer than these outlines would indicate ; so that there is not room ou the plate to 

 show the lull length of this last chamber. 



