242 BULLETIN 11, UNITED STATES NATIONAL MUSEUM. 



The narrow, smooth zooecia, usually with long drawn-out zooecial 

 apertures separated by similarly shaped mesopore spaces and the 

 absence of diaphragms, will serve to distinguish this species from 

 associated small, ramose Trepostomata. The characters of speci- 

 mens from Minnesota are shown in figure 135, while the identity of 

 the Russian examples is apparent from the internal structure illus- 

 trated in figure 136. 



Occurrence. — Not uncommon in the Rhinidictya bed of the Black 

 River (Decorah) shales of Minnesota. Less common in the Was- 

 salem beds (D3) at Uxnorm, Esthonia. 



Plesiotype.— Cat. No. 57314, U.S.N.M. 



Specimens and thin sections from Uxnorm in the collections of 

 the British Museum. 



Genus ERIDOTRYPA Ulrieh. 



Batostomella (part) Ulrich, Geol. Surv. Illinois, vol. 8, 1890, pp. 375, 432. 



Eridotrypa Ulrich, Geol. and Nat. Hist. Surv. Minnesota, vol. 3, pt. 1, 1893, p. 

 264.— NiCKLES and Bassler, Bull. 173, U. S. Geol. Surv., 1900, p. 32.— 

 Bassler, Bull. 292, U. S. Geol. Surv., 1906, p. 29.— Cumings, Thirty-second 

 Ann. Rep. Dep. Geol. Nat. Res. Indiana, 1907, p. 745. — Hennig, Archiv iwc 

 Zool., vol. 4, No. 21, 1908, p. 36. 



Zoarium ramose, branches slender; zooecia more or less obUque, 

 thick-walled, and intersected by diaphragms, which are most numer- 

 ous and most closely set in the earlier portion of the short, mature 

 region; mesopores sometimes numerous, sometimes few, with closely 

 set diaphragms ; acanthopores small, few, or wanting. 



Genotype. — Eridotrypa mutdbilis Ulxich. Middle Ordovician (Black 

 River) of the Mississippi Valley. 



All of the American Black River species of Eridotrypa are repre- 

 sented in the several formations of division ~D, and in the Wesenberg 

 limestone. 



ERIDOTRYPA .fflDILIS (Eichwald). 



Plate 4, figs. 5, 5a; text figs. 137, 138. 



Cladopora sedilis Eichwald, Bull. Soc. Nat. Moscou, No. 4, 1855, p. 457; Lethsea 



Rossica, vol. 1, 1860, p. 404, pi. 24, figs. 12, 13. 

 Monticulipora sedilis Dybowski, Die Chsetetiden Ostbaltischen Silur-Formation, 



1877, p. 98, pi. 3, figs. 5, 5a. 

 Eridotrypa mutabilis Ulrich, Geol. and Nat. Hist. Surv. Minnesota, vol. 3, pt. 1, 



1893, p. 265, pi. 26, figs. 20-32. 



The Wesenberg beds of the Russian Baltic provinces contain 

 abundant specimens of the bryozoan well figured by Eichwald as 

 Cladopora sedilis. The same species occurs less abundantly in the 

 different divisions of formation D. Upon comparison these speci- 

 mens are found to be identical with a very common, wide-spread 

 American species, Eridotrypa mutabilis Ulrich, occurring in the 

 Black River and lower Trenton strata. Dybowski has given good 



I 



