276 



BULLETIISr 11, UNITED STATES NATIONAL MUSEUM. 



Occurrence. — Abundant in the Stictoporella bed of the Black River 

 (Decorah) sbale in Minnesota; rare in the Echinospherites limestone 

 (Cl), near Reval, Esthonia (Cat. No. 57379, U.S.N.M.). 



Fig. 164.— Batostoma fertile circulaee. a and &, stteface of a typical example, x9, with a por- 

 tion, X18; C AND d, TWO tangential sections, X18, FROM DIFFERENT PARTS OF THE MATURE ZONE. 



Black River (Decorah) shales, Minneapolis, Minnesota. (After Ulrich.) 



American specimens of the species are in the collections of the 

 British Museum. 



BATOSTOMA MICKWITZI, new species. 



Plate 10, figs. 3-6; text fig. 165. 



This striking bryozoan may be easily recognized by its unusually 

 robust branches, very large polygonal zooecia and few mesopores. 

 In size of zoarium the species is approached by no other of the known 

 Paleozoic ramose bryozoans, while such unusually large zooecia are 

 found in but few branching species. The small fragment figured 

 on plate 10 averages 25 mm. in diameter, but the more complete 

 zoarium illustrated has branches 40 mm. in diameter, increasing to 

 70 mm. just before a bifurcation. This latter specimen, and another 

 showing the basal portion clearly, indicate that the complete zoarium 

 consisted of a stout, slightly spreading basal portion arising into a 

 short, cylindrical stem, which, after a growth of 60 mm. or more, 

 divided dichotomously. After a growth for a similar distance, each 

 of these divisions branches in the same way, but after this second 

 branching, zoarial growth seems to have stopped. An entire zoarium 

 was therefore probably seldom over 150 mm. in height. 



Surface of a zoarium smooth, maculae scarcely distinguishable 

 with the naked eye, but under a hand lens they are seen to be com- 

 posed of the usual large zooecia and more numerous mesopores. 

 Zooecia angular, unusually large, four to four and one-half in 2 mm. ; 

 an ordinary zooecium averages 0.5 mm. in diameter, while the largest 

 zooecia of the maculae are one-half again as large. Zooecial walls 

 thin; acanthopores apparently seldom developed, probably repre- 

 sented by knot-like structures in the cell angles; mesopores few, 



