278 TJic American Geologist. May, i8yf< 
ences in size and state of preservation, e. g., between certain 
species referred to Tryblidiiii}i and some of the forms referred 
by authors to Stcnotheea it is (Hfficult to describe a satisfac- 
tory difference beyond the fact that one is twenty times the 
size of the other, ahliongh a glance would seem to be suffi- 
cient to separate them. 
The tendency among American paleontologists until re- 
cently was to place many unlike forms in the genus Metop- 
kvna, Phillips, 1836. Since 1872, however, as new forms 
have been studied, distinctions have been made which allow of 
a considerably more complete subdivision. 
The Metoptojiia type is truncated under the apex and has 
a horse-shoe shaped muscle scar. These two points together 
serve to distinguish the genus. No materials in the Taylor's 
Falls collection belong to this genus. 
Lepetopsis, Whitfield, 1882, is from later formations and 
is sufficiently well distinguished from all of the more primitive 
types by its dextrally coiled nucleus. 
See/iella, Billings, 1872, includes patelliform shells with cir- 
cular or oval aperture and subcentral apex. Muscle attach- 
ments form a complete circle so far as known. 
Stenotlieca, (Salter), Hicks, 1872, and PalcEacmiea, Hall 
and Whitfield, 1873, may be divisions worthy of generic dis- 
tinction, but in the present condition of our knowledge they 
can scarcely be given more than sub-generic rank with any as- 
surance, smce surface ornamentation alone cannot be very gen- 
erally accepted as a generic character. 
Co/iehope/tis, Walcott, 1876, is a lobed form and of so 
irregular outline and altogether of so uncertain affinities that 
it need not claim serious attention in a study of the types in 
hand. 
AreJiiimcclla, Ulrich, 1897, presents some perplexities. In 
so far as a continuous muscle attachment coupled with the 
outline of Tryblidiiiui is substantiated by actual specimens 
there seems to be a ])lace for the genus. But since a major- 
ity of the species referred by Ulrich* to this genus do not 
exhibit \\ell marked muscle attachment of any kind and in 
addition conform closely in outline either to Tryblidiuin or to 
*Geol. and Nat. Hist. Surv. of Minn., Final Rep., vol. Ill, part II. 
p. 828. 
