EAELY PALEOZOIC BKYOZOA OF THE BALTIC PEOVINCES. 183 



Zocecial apertures polygonal, with thin walls, often bearing large 

 acanthopores ; six and one-half to seven zooecia in 2 mm. True 

 mesopores practically absent, the mesopore-like spaces seen in 

 tangential sections probably representing young zocecia. 



The most striking feature of thin sections is the presence of large 

 acanthopores, numbering about one to a zo(»cium. These are best 

 shown in tangential sections where they appear as well defined, 

 centrally perforated, circular spots of darker tissue made up of 

 concentric rings. The acanthopores are likewise conspicuous in 

 both vertical and tangential fractures, so that thin sections are not 

 necessary in identifying the species. Vertical sections show rather 

 numerous straight or slightly curved diaphragms, four or five occur- 

 ring in a tube diameter in the more crowded portions, but two to 

 three being the more usual number. 



Apparently the closest ally of the species is the American form 

 Orhignyella lameUosa (Ulrich), from the Richmond (Fernvale) shales 

 of Illinois. The latter grows into lamellate expansions with thin- 

 walled zooecia (eight in 2 mm.), moderately large acanthopores, and 

 straight or curved diaphragms averaging half a tube diameter apart. 

 The different methods of growth, slightly larger zooecia (six and one- 

 half to seven in 2 mm.), larger acanthopores and more numerous 

 diaphragms of the Russian form are relied upon in founding this new 

 species, although its intimate relationship to 0. lameUosa is recognized. 



Occurrence. — Apparently rare in the Wassalem beds (D3) at 

 Uxnorm, near Reval, Esthonia. 



Holotype.— Cat. No. 57271, U.S.N.M. 



British Museum, thin section of the type-specimen. 



ORBIGNYELLA EXPANSA BALTICA, new variety. 



Text fig. 96. 



Cfr. Orhignyella expansa Bassler, Bull. 292, TJ. S. Geol. Siirv., 1906, p. 26, 

 pi. 10, figs. 5-8. 



The specimen upon which this new variety is founded occurs as an 

 incrustation 3 mm. in thickness, upon a Heliolites. Its diameter is 

 about 20 mm., so that it is believed with further growth the zoarium 

 might have become a free, lamellate, epithecated expansion. All of 

 the zocEcial characters save one are as in Orhignyella expansa from the 

 Niagaran (Rochester) shale of North America. The one exception 

 is the small number of cystiphragms in the specimen under discus- 

 sion ajid their comparative abundance in the typical form of the 

 species. The intimate relationship of the species and variety is thus 

 apparent, and the latter represents undoubtedly only an earlier 

 appearance of the species. Making allowance for the possible differ- 

 ence in growth and the variation in development of cystiphragms, the 

 following description of 0. expansa will serve equally well for the 

 recognition of the variety haltica. 



