EAKLY PALEOZOIC BRYOZOA OF THE BALTIC PROVINCES. 71 



described by Ulrich from the Lower Trenton shales of Minnesota. 

 Ulrich's description is as follows: 



Zoarium ramose, very small, the branches cylindrical, 0.5 or 0.6 mm. in diameter, 

 with faint transverse strige or wrinkles over the spaces between the zooecial apertures. 

 The latter are drawn out tube-like, about 0.15 mm. in diameter, and project strongly 

 upward and outward from the surface of the small stems. Their arrangement is in 

 rapidly ascending spiral series, with four or five in 2 mm. As near as can be deter- 

 mined from the material at hand, the zooecial tubes diverge equally to all sides of the 

 branches from an imaginary axis. 



Occurrence. — Common in the Nematopora bed of the Trenton, at 

 Cannon Falls, Minnesota. Apparently rare in the Kuckers shale 

 (C2), Baron Toll's estate, near Jewe, Esthonia. 



Plesiotype.—Csit. No. 57187, U.S.N.M. 



Family IDMONEID.^ Busk. 



This family is represented in the Ordovician and early Silurian 

 rocks by the single genus, Protocrisina, which is possibly incorrectly 

 placed here. Typical Idmoneidse are most abundant in the late 

 Cretaceous and Cenozoic times. 



Genus PROTOCRISINA Ulrich. 



Protocrisina Ulrich, Geol. Surv. Illinois, vol. 8, 1890, p. 369. — Pocta, Syst. Sil. 

 Centre Boheme, vol. 8, pt. 1, 1894, p. 16. — Ulrich, Zittel's Textbook of Paleon- 

 tology (Eng. ed.), 1896, p. 262, text fig. 417. — Nickles and Bassler, Bull. 

 173, U. S. Geol. Surv., 1900, p. 21. 



Protocrisina is a somewhat unusual genus distinguished from all 

 other genera of similar growth by the occurrence of small pores on 

 both sides of the branches. The generic characters are as follows: 



Zoarium consisting of narrow, bifurcating branches, celluHferous 

 on one face only; zooecia subtubular, with prominent, circular aper- 

 tures arranged in intersecting diagonal series; on both faces small pores 

 irregularly distributed. 



Genotype. — Protocrisina exigua Ulrich. Earliest Silurian (Eich- 

 mond) of the United States, Canada, and Sweden. 



Only a single species has hitherto been recognized in this genus, but 

 the collections of the United States National Museum now contain 

 representatives of at least five distinct forms. The genotype was 

 described from specimens obtained in the Richmond group, but the 

 same species or a closely related variety was recognized in the early 

 Trenton strata of New York and Canada. The latter form was 

 described by Hall as Gorgoniaf perantiqua,^ but his description and 

 figures were, like many others of that early period, hardly sufficient 

 for the certain identification of the species. Hall's type-specimen, 



1 Nat. Hist. New York, Pal., vol. 1, 1847, p. 76, pi. 26, figs. 5a, b. 



