46 PALAEONTOLOGY OF THE UPPER MISSOURI. 



eyes. From all the facts, it seems to be much more probable that Anaiomns and 

 Plenrolomarla are identical, than that either belong to the same genns as Scissn- 

 orUa. If they are synonymons, of course Montfort's name Anafovms must be 

 adopted, since it has priority of date. Until their identity is more satisfactorily 

 determined, however, we prefer to retain the name Pleurotomaria. 



The fovmder of this genus divided it into two sections, one including the um- 

 bilicated species, Avith a rounded aperture, and the other those without an umbilicus. 

 These sections have been regarded by other authors as distinct genera, but there 

 are so many intermediate gradations comiecting the umbilicate and imperforate 

 species, that it is scarcely possible this can be regarded as a generic character. It 

 is quite probable, however, that distinct genera have been confounded under the 

 name Pleurotomaria, amongst the fossil species.^ 



The genus Pleurotomaria was introduced at an early period, a number of species 

 having been described from the Lower Silurian Rocks. It is also represented 

 through the succeeding formations, and is particularly abundant in the Coal 

 Measures of the Western States. It attained its greatest numerical development 

 during the Jurassic epoch, and is well represented in the Cretaceous strata ; since 

 the deposition of which it has rapidly declined. At present but two living species 

 are knoAvn, one of which occurs on the coast of Marie Galante, and the habitat of 

 the other is unknown. Both of these species are very similar to some of those 

 found in the Jurassic and Cretaceous rocks. The Pala?ozoic species, as might be 

 expected, retain no traces of their original pearly lustre, though some of those 

 found in the more modem rocks are still nacreous. 



Pletii'otoiiiaria Iiunierosa. 



(Plate I, Fig. 14, a, h.) 

 Pleurotomaria humerosa, Meek & Hatden, Proceed. Acad. Nat. Sci. Phila. Deo. 1858, 262. 



Shell turbinate, or conical subovate ; spire turreted, moderately elevated. Volutions five to five and a half, very 

 convex, more or less obliquely fiattened above, rounded below, and distinctly angular at the outer margin of the 

 flattened upper side. Suture distinct ; columella with a small shallow umbilical perforation. Surface ornamented 

 by about ten rather strong revolving lines, only some three or four of which are visible on the upper whorls below 

 the angle ; on the fiattened upper surface of the whorls there are five to seven additional, much smaller revolving 

 striffi, sometimes obsolete on worn specimens. Aperture suborbicular. 



Length, 0.62 inch ; breadth, 0.50 inch ; apical angle about 62°. 



None of the specimens of this species we have seen have the lip entire, and as 

 the lines of growth are not preserved, it is impossible to determine the nature and 

 position of the labial sinus and spiral band. From analogy, however, we would 

 suppose the band to occupy a position just outside of the angle of the shoulder, 

 and of course the sinus of the lip, in that case, Avould be near the point where it is 

 intersected by this angle. 



This shell seems to bo rather closely allied to Pleurotomaria Yvanii, LeveiUe 



• Hcrmannsen places Plijchomphalus, Agassiz (Germ. Trans. Sowerby's Min. Conch. 1837, 23, 

 222, 310), as a synonym of Pleurotomaria. We arc not well enough acquainted, however, with the 

 type of Prof. Agassiz's genus {Helicina compressa, of Sowerby, from the Lias), which we only 

 know from an examination of Sowcrby's imperfect figures, to express an opinion in regard to its 

 relations. 



