190 THE PALEONTOLOGY OF MINNESOTA. 
(Helopora. 
Section a embraces the species which agree nearest with the type of the genus, 
and all of them, as far as known, belong to Upper Silurian rocks (including the 
Anticosti group). They are distinguished from the species of section } (1) by the 
presence of acanthopores, one of which is commonly situated in each end space ; (2) 
the absence of longitudinal ridges and inter-apertural striz ; (3) the predominance of 
the diagonal arrangement of the zocecial apertures; (4) the absence of a peristome, 
the interspaces being highest midway below the zocecial apertures and sloping into 
them, and (5) the lesser separation of the zocecial apertures longitudinally. 
Under section a I would place H. fragilis Hall, from the Clinton of New York, 
Canada, and probably Ohio; H. bellula, H. armata, and H. nodosa, three species 
described by Billings from the Anticosti group, and H. lindstremi Ulrich, from the 
Upper Silurian of the island of Gotland. 
Section 6 differs from the typical section {1) by the absence of acanthopores; (2) 
the presence of straight or wavy ridges and minor striations of the surface; (3) the 
predominance of the longitudinal and transverse arrangement of the zocecial aper- 
tures; (4) the prominence of the zocecial apertures, especially at the inferior side, 
and (5) in the more ornamental appearance of the segments resulting from the pecu- 
hiarities noted. 
Helopora spiniformis, originally described by me as Arthroclema spiniforme (Jour. 
Cin: Soc. Nat. Hist., vol v, p. 161, 1882), may well be accepted as the type of this 
section. This species is fairly abundant on slabs of “Glade” limestones, at Lebanon, 
LaVergne, and other localities in Tennessee. ‘Figs. 4, 5 and 6, on plate ILI, intro- 
duced chiefly for comparison with H. divaricata, illustrate its principal characters. 
Segments of apparently the same species were collected also in the lower limestones 
at Dixon, Illinois. 
All the other known Lower Silurian species referred to the genus must be 
arranged in section b. These are H. quadrata, n. sp., H. mucronata Ulrich, and H. (?) 
sp. undet., from the Galena shales of Minnesota, the last two with rather striking 
resemblances to certain Cylostomata; H. harrisi James, HH. elegans, n. sp., and 
HH. imbricata Ulrich, from the upper beds of the Cincinnati group of Ohio and 
Illinois; and H. alternata Ulrich, from the Trenton shales of Minnesota. The last 
represents a peculiar type of the section that is again met with, but less strongly 
expressed in the new species H. elegans. 
As is to be expected, it is in section that’ Helopora most nearly approaches the 
other genera of the family. The species of section a represent a further differenti- 
ation of the type. Comparing the former with <Avrthroclema we note a general 
agreement of structure, which, if we knew nothing of the segments of the primary 
and secondary order of that genus, might really be said to amount to identity. But 
