Geology of the St. Croix Dalles. — Berkey. 277 
These two lines of variation are valid morphologic grounds 
for generic distinction and division. They exhibit an un- 
broken series of forms leading to independent but perhaps 
closely related genera. Regarding internal muscle scars it 
is evident that a statement recently made in a work upon this 
subject by Air. Ulrich needs revision. His statement is es- 
sentially as follows:* An examination of all forms on which 
muscle scars are known supports this biologic or structural 
law that: 
1st, all forms in which the muscle scars are interrupted 
are anteriorly (acuminately) excentric. 
2nd, all forms in which the muscle attachment is continu- 
ous are posteriorly (obtusely) excentric, i. e., the apex in- 
clines toward the larger rounded margin of the aperture. 
Notwithstanding this general statement by Mr. Ulrich, I 
must insist that it does not apply to the forms to be described. 
Paired muscle scars have been detected in both lines of varia- 
tion. A few of the obtusely inclined forms show the marks 
so well and are at the same time such typical specimens of 
their own group that no rule of such sweeping generalization 
could be formulated, especially since most primitive types 
show nothing for or against such conclusions. 
Faimal Relationships. — There have been no less than nine 
different generic names proposed for the simple cone-like 
shells which occur fossil in Pahtozoic rocks. Among these 
most all possess characteristics either of form or muscle at- 
tachment sufificiently constant to hold an independent place 
in classification. A few of them, however, seem to be based 
upon characters of too questionable value for generic distinc- 
tions, notwithstanding their apparent value in distinguishing 
species or varieties or individuals. Such a character, for ex- 
ample, is that of surface marking. With the large number 
of specimens in hand it seems to be shown in these forms, as 
has been long known in many others, that fine or coarse stria- 
tion either radially or concentrically or even strong plication 
are characters of comparatively little taxonomic importance. 
Among the obstacles to a correct adjustment is the magni- 
fied apparent discrepancies of type arising from great dififer- 
*Geol. and Nat. Hist. Surv. of Minn., Final Rep., vol. Ill, part II. 
1897, p. 828. 
