1 N VERTEBRATE PALvKONTOU X ; Y. 295 



in rucks of differenl ages, even back to the Carboniferous. It is probable, 

 however, thai many of those found in the Cretaceous and Jurassic rocks will 

 prove to have the internal characters of Anisomyon; though some of them 

 seem, even from internal characters, to belong to this genus 



The genus Acmcea also occurs in the Tertiary rocks, and perhaps attains 

 its greatest development in our existing seas The living species range from 

 low tide to thirty fathoms, and occur on the easteim and western shores of 

 North America, Australia, Ac. 



1 merely followed Gray, H. and A. Adams, and other high authorities 

 in using the name Tectura for this genus in former publications, because I had 

 not thoroughly' traced out its synonymy. Since doing so, I am satisfied that 

 Mr. Dall is right in adopting the oldest name, Acmcea, for it. 



A c in a; a occiden talis, M. & H. 



Plate 18, figs. 3, a, b. 



Orbicula (limit.), Owen (1852), Geological Report Wisconsin, Iowa, and Minnesota, pi. 8, fig. 11. 

 Capulus occidentalis, Hall and Meek, Mem. Am. Acad. Arts and Sei. Boston, V (u. s.), 385, pi. 1, fig. 13. 

 Tectura '. occidentalis, Meek and Hayden (1860), Proceed. Acad. Nat. Sei. Philad., XII, 4-23.— Meek (1804), 

 Smithsonian Check-List N. Atu. Cret. Fossils, 17. 



Shell suborbicular, much depressed, and very thin; summit slightly in 

 advance of the middle, apparently obtusely pointed, and directed slightly 

 toward the front ; surface smooth, or only marked by fine, obscure lines of 

 growth. Faint radiating lines are also seen on the surface of internal casts, 

 but they are very obscure and irregular, and appear to be in no way con- 

 nected with the surface-markings. 



Greater diameter, 0.49 inch; breadth, 0.47 inch; height, 0.11 inch. 



Although I am in doubt respecting the generic relations of this shell, 

 it is clearly not congeneric with the recent typical forms of Capulus. In the 

 first place, it is a much more depressed shell than any true Cupulas, and has 

 its apex much less attenuated, and not in the least degree spiral or incurved. 

 Again, it differs in having its apex and the open end of its horse-shoe-shaped 

 muscular scar (which latter is clearly seen on some of the internal casts) 

 directed toward the same end of the shell, while the reverse is the case in 

 Capulus. It will lie readily distinguished from any of the other known 

 patelliform shells found in the Upper Missouri rocks, by its more depressed 

 form, as well as by having its muscular scar uninterrupted posteriorly. 



