46 PALAEONTOLOGY OF THE UPPER MISSOURI. 



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

 Pleurotomaria are identical, than that either belong to the same genus as /Scissn- 

 rella. If they are synonymous, of course Montfort's name Anatomus 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 founder of this genus divided it into two sections, one including the um- 

 bilicated species, with 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 connecting 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. 1 



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 known, 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 Palaeozoic species, as might be 

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

 found in the more modern rocks are still nacreous. 



Pleurotomaria humerosa. 



(PLATE I, Fig. 14, a, 6.) 

 Pleurotomaria humerosa, MEEK & HATDEN, Proceed. Acad. Nat. Sci. Pliila. Doc. 1858, 262. 



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

 convex, more or less obliquely flattened 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 flattened upper surface of the whorls there are five to seven additional, much smaller revolving 

 striae, sometimes obsolete on worn specimens. Aperture snborbicular. 



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, would be near the point where it is 

 intersected by this angle. 



This shell seems to be rather closely allied to Pleurotomaria Yvanii, Leveille 



1 Hcrmannsen places Plychomphalus, Agassiz (Germ. Trans. Sowerby's Min. Gonch. .1837, 23, 

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

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

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

 relations. 



