332 



BULLETIN 11, UNITED STATES NATIONAL MUSEUM. 



apertures in young examples more or less oblique, ovate, and separated 

 by numerous mesopores. With age, the apertures become more 

 direct and angular, with less numerous mesopores, as illustrated in 

 figure 207 e. Seven to eight zooecia in 2 mm. The American speci- 

 mens occasionally show apertures with closures marked by faint 

 lines radiating from a small central perforation. The internal char- 

 acters are sufficiently illustrated in figure 207 g and Ti to require no 

 description. 



Occurrence. — Common in the Phylloporina bed of the Black River 

 (Decorah) shales, and in the Clitambonites bed of the lowest Trenton 

 at St. Paul and Cannon Falls, Minnesota. Apparently rare in the 

 Kuckers shale (C2) at Eeval, Esthonia (Cat. No. 57468, U.S.N.M.). 



Represented in the collections of the British Museum by specimens 

 from American localities. 



HALLOPORA UNDULATA (Ulrich). 



Text fig. 208. 



Callopora undulata Ulrich, Fourteenth Ann. Rep. Geol. Nat. Hist. Surv. Minne- 

 sota, 1886, p. 95; Geol. Nat. Hist. Surv. Minnesota, vol. 3, pt. 1, 1893, p. 279, 

 pi. 22, figs. 24-31. 



The slender, ramose branches with rather large rounded monticules 

 and the internal structure shown in figure 208, are sufficient to sepa- 

 rate this neat little Hallopora from other species of the genus. The 



Fig. 208.— Hallopoka undulata. a and b, typical specimens op the natural size; c, surface of 



OKIGINAL OF FIGUEE 6, X9; d, VERTICAL SECTION, X18. BLACK ElVEE (DeCOEAH) SHALES, ST. PAUL, 



Minnesota. (After Ulrich.) 



Russian examples referred to the species differ only in having a 

 slightly less robust zoarium, a difference which would undoubtedly 

 be eliminated with larger collections. 

 Ulrich's description is quoted below: 



Zoarium ramose, branches slender, averaging about 2.5 or 3 mm. in diameter, divid- 

 ing dichotomously at intervals of 10 mm. or more. Surface with rather large, rounded 

 monticules, that usually coalesce laterally, forming transverse ridges, or more or less 

 complete annulations, 5 in 10 mm. In some fragments and portions of others the 



