364 THE PALEONTOLOGY OF MINNESOTA. 



(.Orbiciiloidea lamellostt. 



The species now referred to this genus were formerly regarded as congeneric 

 with the recent species Distinct striata Schumacher, the type of Distinct. So far as 

 known that' genus is restricted to a single species, all the other recent forms formerly 

 referred to Distinct now belonging to Discinisca, Dall. Other Palaeozoic subgenera of 

 the same type of structure as Orbiculoidea are ScMzotreta, Kutorga, (Ehlertella, Lind- 

 strcemella and Rcemerella, Hall. 



ORBICULOIDEA LAMELLOSA Hall? 



PLATE, XXIX, FIG. 25. 



1847. Orbicula lamellosa HALL. Palaeontology of New York, vol. i, p. 99, pi. xxx, fig. 10 (not 



Orbicula lamellosa Broderip, 1833= Discinisca lamellosa). 

 1855. Orbicula truncata EMMONS. American Geology, part ii, p. 200, flg. 62. 

 1860. Discina truncata 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. ivE, flg. 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 [dorsal] 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." 



