EARLY PAIiEOZOIC BRYOZOA OP THE BALTIC PROVINCES. 113 



successive layers of polypides are often developed, one directly over 

 the other, in a continuous tube; and, fourth, in that whenever a 

 zoarium attains an uninterrupted width of more than. 8 mm., it 

 exhibits clusters of cells differing more or less, either in size or eleva- 

 tion, from the average zooecia. 



The generic and even specific representation of the Cryptostomata 

 in the Baltic Ordovician and early Silurian strata is exceptionally 

 like that in the corresponding beds of America. 



Family PTILODICTYONIDiE Zittel. 



Zoarium bifohate, composed of two layers of zooecia, grown together 

 back to back, usually jointed, at least at the base, and forming leaf- 

 like expansions or compressed branching or inosculating stems; 

 mesotheca without median tubuli; zooecia usually have hemisepta 

 and semielliptical orifices ; apertures usually ovate, surrounded either 

 by a sloping area or a ring-like peristome; vestibules separated by 

 thick walls. 



Genus PTILODICTYA Lonsdale. 



Ptilodictya Lonsdale, Murcliison's Silurian System, 1839, p. 676. — EiCHWjiiD, 

 Lethsea Rossica, vol. 1, 1860, p. 387. — Ulrich, Joum. Cincinnati Soc. 

 Nat. Hist., vol; 5, 1882, pp. 151, 162.— Foerste, Bull. Sci. Lab. Denison 

 University, vol. 2, 1887, p. 155. — Hall and Simpson, Nat. Hist. New York, 

 Pal., vol. 6, 1887, p. xix.— Ulrich, Geol. Surv. Illinois, vol. 8, 1890, p. 390; 

 Geol. Nat. Hist. Surv. Minnesota, vol. 3, pt. 1, 1893, p. 163; Zittel's Text- 

 book of Paleontology (Eng. ed.), 1896, p. 279. — Simpson, Fourteenth Ann. 

 Rep. State Geologist New York for the year 1894, 1897, p. 541. — Nickles and 

 Bassler, Bull. 173, U. S. Geol. Surv., 1900, p. 45.— Hennig, Archiv. fur 

 Zool., K. Sven. Vet.-Akad. Stockholm, vol. 2, No. 10, 1905, p. 16.— Cumings, 

 Thirty-second Ann. Rep. Dep. Geol. Nat. Res. Indiana, 1907, p. 754. 



Escharo-pora (not Hall, 1847) Hall, Twenty-sixth Ann. Rep. New York State 

 Mus., 1874, p. 99; Thirty-second Ann. "Rep. New York State Mus., 1879, p. 161. 



Heterodictya Nicholson, Geol. Magazine, new ser., vol. 2, 1875, p. 33; Pal. Prov. 

 Ontario, 1875, p. 79.— Miller, North Amer. Geol. and Pal., 1889, p. 309. 



Zoarium, a simple, unbranched, lanceolate or falciform frond, 

 narrow or wide, which articulates with a small basal expansion; in 

 the young condition the zoarium consists of longitudinally arranged 

 narrow, oblong-quadrate zooecia, new zooecia of different width and 

 arrangement being, added subsequently on each side; walls of ves- 

 tibules thick, solid, and with a double row of minute dots. 



Genotype. — Flustra lanceolata Lonsdale. Silurian of Europe. 



With the Richmond group assigned to the Silurian, the genus 

 Ptilodictya becomes restricted almost entirely to this period, the 

 single exception being a species in the Lower Devonian of Ontario. 

 Ptilodictya is closely related to EscJiaropora and differs most obviously 

 in the arrangement of its zocecia in parallel longitudinal rows. 



