Geology of the St. Croix Dalles. — Bcrkcy. zyg 
Sce/iella, there would seem to be reason for setting the term 
aside for the present in doubt. 
TryblidiiDn, Lindstrom, 1880, is a well defined genus, 
which as shown by known materials comprises patelliform 
shells with oval or anteriorly acuminate aperture and with 
muscle scars forming a circle of six (or more) pairs. The 
apex of the shell is directed anteriorly, i. e., toward the acum- 
inate margin of the aperture. 
Helcio/iopsis, Ulrich, 1807. '^^ based upon but one charac- 
ter, i. e., fine radiating striae upon the surface of the shell. It 
is clearly a Tryblidiuiii. The distinction is certainly of no 
more than specific value. 
Another group of which a large number of spechnens have 
been collected during work upon the "Geology of the St. 
Croix Dalles" comprises conical shells with the apex bent 
backward. The curve from apex to posterior (broader) mar- 
gin is therefore concave instead of convex as in 'rryblidiii/ji. 
Muscle scars are similar to Tryblidiuni as indicated on the 
casts. This group is in. fact united with 7}^i'/V/ <■/////// through T. 
iretilaterale, n.sp.,hi\i is separated into a new genus because 
of the direction of development which is toward F.eevlioiiipha- 
liis instead of toward Patella and on account of the radically 
dififerent form which at once develops in this line. 7". ree- 
tilaterale again when compared with Hypseloeonus ap- 
proaches H. eyli/idrifus, n. sp., most closely which is onlv 
slightly acuminate and excertric and i;.■^ musculature is un- 
known. H. eyli/idricas differs from Hyolithcs chiefly in its 
direct non-oblicnie and oval a|)erture 7'. reetilaterale has 
the oblique aperture but not the triangular cross section of 
Hyolitlies while Hypseloeonus, recnrviis, var. triangu'atus, has 
a suggestion of the triangular outline l)ut not the txpical 
aperture. There seems to be indicated altogether a primi- 
tive relationship between the Patellidcc, R//o/iiplialidce, and 
perhaps also a more remote relat.'onshij) to the Pteropoda. 
Description of Spkcies. 
OASTKOPOnA. 
Genus Tryblidium, Lindstrom, 1880. 
In conformity with the fact.s just noted bearing upon specific varia- 
bility, it is deemed best to include a greater range of forms within this 
genus than has been at times customary. 
