152 BULLETIN 77, UN^ITED STATES NATIONAL MUSEUM. 



striated surface, strong spines, and indistinct zooecial apertures, is 

 shown in figures 73 a, h. Figure 73 a is especially interesting in illus- 

 trating not only the terminal points of attachment but also a large 

 lateral socket. 



As noted in a previous description, this species and Helopora 

 divaricata are liable to be confused. Comparison of figures 72 and 73 

 wiU show differences in zooecial characters, but the best and surest 

 method of distinction is the discovery of a primary segment of the 

 ArtJiroclema bearing the lateral articulation. 



Occurrence. — The isolated segments of this species are abundant in 

 the Nematopora bed of the lower Trenton limestone at Cannon Falls 

 and St. Paul, Minnesota. In Russia the species so far has been 

 observed only in the Glauconite limestone (B2) at Reval, Esthonia 

 (Cat. No. 57247, U.S.N.M.). 



Represented in the collections of the British Museum by specimens 

 from American localities. 



Genus SCEPTROPORA Ulrieh. 



Sceptropora Ulrich, American Geologist, vol. 1, 1888, p. 228; Conti". Micro-Pal. 

 Cambro-SiL, pt. 2, 1889, p. 46; Geol. Surv. Illinois, vol. 8, 1890, p. 400; 

 Zittel's Textbook of Paleontology (Eng. ed.), 1896, p. 281.— Simpson, Four- 

 teenth Ann. Rep. State Geologist of New York for the year 1894, 1897, p. 

 548.— NiCKLES and Bassler, Bull. 173, U. S. Geol. Surv., 1900, p. 43. 



Sceptropora was founded upon a single species, S. facula from the 

 Richmond rocks of Manitoba, Canada, but has since been noted in 

 these strata at many other loeaHties. The discovery of numerous 

 undoubted examples of S. facula in the Borkholm limestone is in line 

 with other evidences showmg the wide distribution of this Richmond 

 fauna. The generic characters of Sceptropora are as follows : 



Zoarium of segments which become much expanded in their upper 

 portion and at the top have a socket for the articulation of the next 

 segment; lower portion striated, without apertures; upper part with 

 apertures all around; apertures subovate, in linear series between 

 longitudinal ridges. 



Genotype. — Sceptropora facula Ulrich. Earhest Silurian (Rich- 

 inond) of North America and Russia. 



Since the pubHcation of the type species of the genus, a second 

 specimen from the Clinton rocks of Canada has been described by 

 Ulrich. The Russian coUections now afford a third species, differing 

 decidedly from the genotype but being not far removed in its charac-. 

 ters from the Clinton form. 



