EAEX.Y PALEOZOIC BEYOZOA OF THE BALTIC PKOVINCES. 149 



Genus HELOPORA Hall. 



Helopora Hall, Amer. Journ. Sci., ser. 2, vol. 11, 1851, pp. 388,389; Nat. Hist. 

 New York, Pal., vol. 2, 1852, p. 44.— Billings, Cat. Sil. Foss. Anticosti, 1866, 

 p. 86. — Ulrich, American Geologist, vol. 1, 1888, p. 231; Geol. Surv. Illinois, 

 vol. 8, 1890, pp. 401, 642; Journ. Cincinnati Soc. Nat. Hist., vol. 12, 1890, 

 p. 191; Geol. and Nat. Hist. Surv. Minnesota, vol. 3, pt. 1, 1893, p. 189; 

 Zittel's Textbook of Paleontology (Eng. ed.), 1896, p. 280.— Simpson, Four- 

 teenth Ann. Rep. State Geologist New York for the year 1894, 1897, p. 548. — 

 NiCKLES and Bassler, Bull. 173, TJ. S. Geol. Surv., 1900, p. 42. — Hennig, 

 ArchivfurZool., K. Sven. Vet.-Akad. Stockholm, vol. 3, No. 10, 1906, p. 18. 



Helopora was founded upon an early Silurian species, Helopora 

 fragilis Hall, from the Clinton and Niagara rocks of New York and 

 Canada, and has its best and most typical representation in this time. 

 Unquestionable species of the genus are, however, present in the 

 Middle Ordovician, and the discovery of the following H. divaricata 

 Ulrich in the Kuckers shale is an interesting extension of the geo- 

 graphic range of the genus and species. 



Helopora differs from Artlirostylus in that its segments are gen- 

 erally much larger and bear zooecial apertures on all sides. The ter- 

 tiary segments of species of Arthroclema have much in common with 

 Helopora, and care must be exercised in separating them. 



Genotype. — Helopora fragilis Hall. Silurian (Clinton) of New York 

 and Canada. 



HELOPORA DIVARICATA Ulrich, 



Text fig. 72. 



Helopora divaricata Ulrich, Fourteenth Ann. Rep. Geol. Nat. Hist. Surv. Minne- 

 sota, 1886, p. 59; Geol. and Nat. Hist. Surv. Minnesota, vol. 3, pt. 1, 1893, 

 p. 191, pi. 3, figs. 1-3. 



A number of isolated segments of this species have been observed 

 on the thin slabs of limestone in the Kuckers shale. None of these 

 happens to be as large as the American type of the species, and as 

 the zooecial apertures increase slightly in diameter with the size of 

 the segment itself, the Russian examples naturally have smaller aper- 

 tures. However, these specimens may be matched in the American 

 collection, so that there can be little doubt regarding the identity of 

 the Russian examples. 



The jointed zoarium of Helopora divaricata is made up of sub- 

 cylindrical segments obtuse at both ends, about 7 mm. long and 

 bearing zooecial apertures, of which there are from six to eight rows, 

 opening on all sides. The diameter of a single segment ranges from 

 0.5 to 0.9 mm. The apertures number 12 in 5 mm., measuring 

 lengthwise, are comparatively large, oblique, and ovate in shape, and 

 are situated between strong longitudinal ridges. The posterior border 

 of the aperture is thick and prominent, and slopes backward into the 



