150 



BULLETIN 77, UNITED STATES NATIONAL MUSEUM. 



Fig. 72.— Helopoea divabicata. a, fragment 

 natural size and xv; 6, portion of the same, 

 x18; c, transverse section, x18. black river 

 (Decorah) shale, Minneapolis, Minnesota. 

 (After Uleich.) 



aperture next below. This border continues along the sides of the 

 apertures as two diverging ridges, extending to the summit of the 



longitudinal crests, as shown in 

 the accompanying figure. 



The segments of H. divaricata 

 are so much larger than those of 

 the species of Arthrostylus that 

 comparisons are not needed. 

 They might, however, be con- 

 founded with the tertiary seg- 

 ments of Arthroclema armatum. 

 Comparisons of the two species 

 are given in the description of 

 the latter. 



Occurrence. — Helopora divari- 

 cata is not uncommon in the 

 Stictoporella bed of the Black 

 River (Decorah) shales at Min- 

 neapolis and other localities in 

 Minnesota, and is equally abun- 

 dant in the Kuckers shale (C2), Baron Toll's estate, Esthonia (Cat. 

 No. 57246, U.S.N.M.). 



British Museum, one specimen from the Kuckers shale, Baron Toll's 

 estate. 



Genus ARTHROCLEMA Billings. 



Arthroclema Billings, Pal. Foss., vol. 1, 1862, p. 54.— Uleich, Fourteenth Ann. 

 Rep. Geol. Nat. Hist. Surv. Minnesota, vol. 5, 1882, p. 151; American Geolo- 

 gist, vol. 1, 1888, p. 232; Journ. Cincinnati Soc. Nat. Hist., vol. 12, 1890, p. 

 192; Geol. Surv. Illinois, vol. 8, 1890, p. 400; Geol. and Nat. Hist. Surv. 

 Minnesota, vol. 3, pt. 1, 1893, p. 197; Zittel's Textbook of Paleontology (Eng. 

 ed.), 1896, p. 281. — Simpson, Fourteenth Ann. Rep. State Geologist of New 

 York for the year 1894, 1897, p. 546. — Nickles and Bassler, Bull. 173, 

 U. S. Geol. Surv., 1900, p. 42. 



Zoarium of segments celluliferous on all sides as in Helopora, but 

 differing in being articulated not only terminally but also laterally in 

 a pinnate manner. The first row of segments and the first set of 

 branches from these are known as the primary and secondary seg- 

 ments and bear articular surfaces on the sides as well as on the ends. 

 The tertiary or last set is articulated only at the end, and the seg- 

 ments of this set must, therefore, have great resemblance to those of 

 Helopora. Since the zocecial structure in each set of segments is 

 practically the same, and all three sets usually occur at the same 

 locality, no especial care is needed in distinguishing them. 



Genotype. —Arthroclema pulcTiellum Billings. Middle Ordovician 

 (Trenton) of Canada. 



