THE PALEONTOLOGY OF MINNESOTA. 
lOrbiculoidea lamellosa. 
The species now refen-ed to this genus were formerly regarded as congeneric 
with the recent species Discina striata Schumacher, the type of Discina. So far as 
known that genus is restricted to a single species, all the other recent forms formerly 
referred to Discina now belonging to Discinisca, Dall. Other Palaeozoic subgenera of 
the same type of structure as Orhiculoidea are Schizotreta, Kutorga, (Ehlertella, Lind- 
stroemella and Roemerella, Hall. 
Orbiculoidea lamellosa Hall? 
PLATE, XXIX. FIG. 2.5. 
1847. Orbicula lamellosa Hall. Palfcontology of New York, vol. i, p. 99, pi. xxx, flg. 10 (not 
Orbicula lamellosa Broderip, 1833=Z)isctn»;sca lamellosa). 
1855. Orbicula truneata Emmons. American Geology, part ii, p. 200, flg. 62. 
1860. Discina truneata Emmons. Manual of Geology, p. 99. 
1862. Discina circe Billings. Palaeozoic Fossils, vol. i, p. 51, flg. 125. 
1890. Orbiculoidea lamellosa Hall. Palaeontology of New York, extract of vol. viii, pi. ia'E, fig. 12. 
Original description: "Orbicular, depressed; apex small, but little elevated, 
situated about one-third the breadth of the shell from the margin; surface marked 
by elevated lamelli-form concentric lines or ridges." 
The following is Mr. Elkanah Billings' description of Discina circe, a species now 
regarded as a synonym of 0. lamellosa: 
"Circular; lower [ventral] valve with the apex central or nearly so; peduncular 
groove acutely oval, extending from the apex about two-thirds the distance to the 
margin. The foramen is probably situated at the outer extremity of the groove, 
but it cannot be seen in the specimen examined. The upper [dorsalj valve (sup- 
posed to be that of this species) has the apex situated about one-third the semi- 
diameter from the margin. In both valves the apex is smooth. 
"Surface with rather strong, sublamellose concentric striae, which become 
more distant and coarser from the apex outwards. At the margin there are four 
or five ridges in one line, but next to the apex double that number in the same 
space. The ridges are somewhat irregular, being in some places slightly undulated, 
and occasionally branched, two or more running into one. The grooves are rather 
wider than the ridges, and the lamellose aspect of the latter appears to be due to 
their being more abruptly elevated on the inner side, or the side towards the apex, 
than on the outside. 
"Width of the specimens of the lower valves examined, nine lines; length of 
peduncular sulcus, three and one-fourth lines; width of the same, one-half line. 
Another specimen (an upper valve) is seven lines wide. 
" The lower [ventral] valve is depressed, conical and appears to have been about 
two lines in hight, but as it is somewhat distorted by pressure, the true elevation can- 
not be determined. The upper [dorsal] valve seems to be less convex than the lower.'' 
