EARLY PALEOZOIC BRYOZOA OP THE BALTIC PROVINCES. 309 



cies. In order to show all of the characters seen in such sections, I 

 have introduced figures of sections prepared from a young zoarium 

 and a portion of a section of an old example (figs. 189 & and 190 c). 

 The arrangement of diaphragms is essentially the same in both. 

 In the immature zone the diaphragms are complete and are at dis- 

 tances of two or more tube diameters from each other. In the bend 

 from the immature to the mature zone, a few semidiaphragms are 

 developed, wliile in the mature zone proper a few scattered complete 

 diaphragms may be seen. The tabulation and wall structures of the 

 mesopores is as usual in members of this family. In the earher stages 

 the walls are moniliform and diaphragms are inserted regularly; 

 later the diaphragms become more numerous and in addition are 

 thickened and appear irregularly placed because of the deposition of 

 layers of tissue upon them. In old examples this tissue is so abun- 

 dant at the surface that the interzooecial spaces are closed entirely. 

 The closely tabulated mesopores with this deposit of tissue gave rise 

 to Dybowski's idea of a ''coenenchyma" separating the corallites 

 or ordinary zooecia. 



In its specific character Dittopora colliculata is approached by no 

 described species. The large, oval, constricted zooecia, conspicuous 

 acanthopores, closed interzooecial spaces with a zoarium of cylindrical 

 stems, made it easy of recognition without considering the marked 

 internal characters. 



Occurrence. — Very abundant in the Wassalem beds (D3) at Uxnorm, 

 Gut Sack, and Wassalem, Esthonia. Rare in the Wesenberg lime- 

 stone (E) at Wesenberg; in the Kuckers shale (C2), Baron Toll's 

 estate, and in the lowest part of the Lyckholm hmestone (Fl) at 

 Kertel and Hohenholm, island of Dago. Several specimens were 

 collected by Doctor Bather in the Chasmops limestone, south of 

 Bodahamn, island of Oeland. 



Plesioty pes. —Cat. Nos. 57403 to 57409, U.S.N.M. 



Specimens and thin sections from the Wassalem beds at Uxnorm, 

 and from the Chasmops limestone, island of Oeland, are in the collec- 

 tions of the British Museum. 



Genus MONOTRYPA Nicholson. 



Monotrypa Nicholson, Paleozoic Tabulate Corals, 1879, p. 293; Genus Monti- 

 culipora, 1881, pp. 102, 168. — Ulrich, Journ. Cincinnati Soc. Nat. Hist., 

 vol. 5, 1882, p. 153.— Hall and Simpson, Nat. Hist. New York, Paleontology, 

 vol. 6, 1887, p. xiii.— Ulrich, Geol. Sui-v. Illinois, vol. 8, 1890, p. 379; Geol. 

 and Nat. Hist. Surv. Minnesota, vol. 3, pt. 1, 1893, p. 303; Zittel's Textbook 

 of Paleontology (Eng. ed.), 1897, p. 275.— Simpson, Fourteenth Ann. Rep. 

 State Geologist of New York for the year 1894, 1897, p. 581.— Nickles and 

 Basslee, Bull. 173, U. S. Geol. Surv., 1900, p. 36.— Bassler, Bull. 292, U. S. 

 Geol. Surv., 1906, p. 46.— Hennig, Archiv. fur Zool., vol. 4, No. 10, 1908, 

 p. 46. 



Ptychonema Hall and Simpson, Nat. Hist. New York, Pal., vol. 6, 1887, pp. xiv, 

 14.— Simpson, Fourteenth Ann. Rep. State Geologist of New York for the 

 year 1894, 1897, p. 583. 



