I114 The American Naturalist. [December, 
tremity are the capulus-like forms, similar to those described by 
White and Whitfield as Platyceras bivolve. 
The synonymy of the genus is as follows: 
1842. Platyostoma, Conrad, Jour. Acad. Nat. Sci., Phila., Vol. 
VIII., p. 275. (Not Klein, 1753; nor Meigen, 1803; nor Agas- 
siz, 1829.) 
1859. Strophostylus Hall, Pale. N. Y., Vol. IIL, p. 303. 
1885. Platystoma Zittel, Hand. der Palæ., Band I., 2 Ab., p. 217. 
1887. Diaphorostoma Fisher, Man. de Conchl., p. 756. 
The following species are regarded as members of the Stro- 
phostylus group. Of the species originally described under this 
genus as understood by Hall, and under Platystoma, several have 
not as yet been sufficiently examined to determine definitely their 
generic position. The majority of those omitted, however, are 
mere internal casts or fragmentary individuals, which have no 
value whatever morphologically ; and from a geologic standpoint 
possess only doubtfully generic interest. 
Strophostylus andrewsi Hall. 
Platystoma arenosum Conrad. 
Platyceras bivolve White and Whitfield. 
Platyceras billingsi Hall. 
Strophostylus cancellatus Meek and Worthen. 
Strophostylus cyclostomus Hall. 
Strophostylus elegans Hall. 
Strophostylus fitchi Hall. 
Platystoma lineatum Conrad. 
Strophostylus matheri Hall. 
Platystoma niagarense Hall. 
Strophostylus obtusus Hal. 
Platystoma peoriense McChesney. 
Platystoma pleurotoma Hall. 
Platystoma strophium Hall. 
Strophostylus transversus Hall. 
Platystoma turbinatum Hall, 
Strophostylus unicus Hall. 
Pleurötomaria unisulcata Conrad. 
Strophostylus varians Hall. 
Platystoma ventricosum Conrad. 
