EAELY PALEOZOIC BEYOZOA OF THE BALTIC PROVINCES. 251 



adjacent zooecia and mesopores are obscured by amalgamation, which, 

 in thin sections, gives them the aspect of a soUd, indivisible structure. 

 In vertical sections the diaphragms of the zooecia are noted to average 

 twice their own diameter apart in the immature zone, and to be 

 sometimes entirely wanting in the mature region. In the mesopores, 

 diaphragms are numerous and are usually about their own diameter 

 apart. 



The club-shaped zoarium pointed at the base, large zooecia, and 

 numerous as well as large acanthopores studding the surface, are 

 specific characters which will not allow the present form to be con- 

 founded with any associated bryozoan. Liodema spineum, from the 

 Orthoceras limestone, has a somewhat similar zooecial structure, but 

 here the zoarium is regularly ramose, the zooecia are larger, and the 

 acanthopores are much stronger. 



Occurrence. — Apparently rare in the Jewe limestone (Dl), Baron 

 TolFs estate (Cat. No. 57327, U.S.N.M.); in the Kegel beds (D2) at 

 Habbinem (Cat. No. 57325 U.S.N.M.) ; and in the Wassalem beds (D3) 

 at Uxnorm (Cat. No. 57326 U.S.N.M.), Esthonia. 



Holotype.— Cat. No. 57325, U.S.N.M. 



British Museum, thin section of specimen from the Jewe limestone, 

 Baron Toll's estate. 



Genus ORBIPORA Eiehwald, 



Orhitulites Eichwald, Zool. Spec, vol. 1, 1829, p. 179. — Milne-Edwards, Hist. 

 Nat. des Corall., vol. 3, 1860, p. 271 (name preoccupied). 



Orbipora Eichwald, Bull. Soc. Nat. Moscou, vol. 29, 1856, p. 92; Lethsea Rossica, 

 vol. 1, 1860, p. 484. — Dybowski, Die Chaetetiden der Ostbaltischen Silur- 

 Form., 1877, p. 57. — Nicholson, Genus Monticulipora, 1881, p. 24. — Waagen 

 and Wentzel, Pal. Indica, ser. 13, 1886, pp. 874, 876. — Nickles and 

 Basslee, Bull. 173, U. S. Geol. Surv., 1900, p. 333. 



This generic name has failed of recognition in the more recent 

 work on Paleozoic Bryozoa, and I would not have thought it advisable 

 to resurrect it had not the genotype, Orhipora distincta, proved to belong 

 to an apparently unoccupied structural division. A detailed descrip- 

 tion of both the genus and type species was given by Dybowski in 1877, 

 but neither his work nor that of his predecessors can be considered of 

 much value for present day purposes. Each writer's definition of 

 the genus has proved so different that now the only method of 

 determining the real generic characters is by a reinvestigation of the 

 types or of authentic specimens of the genotype. Dybowski had 

 access to such specimens, but, although his description and figures in 

 connection with those previously pubhshed by Eichwald were suf- 

 ficient to identify the species with some certainty, they gave little 

 information regarding the generic characters. His ideas concerm'ng 

 the genus were such that he referred here two new ramose species, 0. 

 arhorescens and 0. panderi, the former belonging to a genus still unde- 

 92602°— Bull. 77—11 ^18 



