190 THE PALEONTOLOGY OF MINNESOTA. 



LHelopora. 



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 b (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 striae ; (3) the predominance of 

 the diagonal arrangement of the zooecial 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 zooecial apertures longitudinally. 



Under section a I would place //. fragilis Hall, from the Clinton of New York, 

 Canada, and probably Ohio ; H. bellula, ff. armata, and H. nodosa, three species 

 described by Billings from the Anticosti group, and H. lindstraemi Ulrich, from the 

 Upper Silurian of the island of Gotland. 



Section b 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 zooecial aper- 

 tures ; (4) the prominence of the zooecial apertures, especially at the inferior side, 

 and (5) in the more ornamental appearance of the segments resulting from the pecu- 

 liarities noted. 



Helopora spiniformis, originally described by me as Arthroclemtt spiniforme (Jour. 

 Gin. 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 III, intro- 

 duced chiefly for comparison with II. 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.(1) 

 sp. undet., from the Galena shales of Minnesota, the last two with rather striking 

 resemblances to certain Cylostomata ; H. harrisi JameSj //. elegans, n. sp., and 

 //. imbricata Ulrich, from the upper beds of the Cincinnati group of Ohio and 

 Illinois ; and H. alternuta 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 //. elegans. 



As is to be expected, it is in section b 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 Arthrodema 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 



