132 



BULLETIISr 11, UNITED STATES NATIONAL MUSEUM. 



tinguislied from the conamon Minnesota species Rhinidictya mutabilis, 

 and a second closely related species from the Wesenberg limestone, 

 differing mainly in having much larger zocecia. In America the genus 

 has a rather prolific representation both in species and in specimens. 

 Wliile specimens are not infrequent in the Russian strata mentioned 

 above, the scanty specific development is rather unusual. 



Genotype. — RJiinidictija nicholsoni Ulrich. Middle Ordovician of 

 the United States and Canada. 



RHINIDICTYA MUTABILIS (Ulrich). 



Text fig. 56. 



Stictopora mutabilis (part) Ulrich, Fourteenth Ann. Rep. Geol. Nat. Hist. Surv. 



Minnesota, 1886, p. 66. 

 Stictopora mutabilis Miller, North Amer. Geol. and Pal., 1889, p. 324, fig. 517. — 



Ulrich, Geol. Surv. Illinois, vol. 8, 1890, p. 304, figs. 2 c-h. 

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



1893, p. 125, pi. 6, figs. 1-6, 12, 13; pi. 7, figs. 10-23, 25-28; pi. 8, figs. 1-3.— 



Whiteaves, Pal. Foss., vol. 3, 1897, p. 240. — Saedeson, Journ. Geol., vol. 



9, 1901, p. 155, pi. B, figs. 4-6. 

 Stictopora mutabilis var. minor Ulrich, Fourteenth Ann. E.ep. Geol. Nat. Hist. 



Surv. Minnesota, vol. 3, pt. 1, 1886, p. 67. 



Fig. 66. — ^Rhinidictya mtjtabiiis. a, a toung specimen, desceibed as variety MmoR, natural size; 



6, THE TTSUAL MATURE SPECIMEN; C, SURFACE VIE"W, X9, OF A YOUNG EXAMPLE; d, SEVERAL APERTURES 

 OF THE SAME, X35; 6, SURFACE OF ORIGINAL OF FIGURE 6, X9. BLACK ElVEE (DECORAH) SHALES, MINNE- 

 APOLIS, Minnesota. (After Ulkich.) 



This is a most abundant bryozoan in the Black River (Decorah) 

 shales of Minnesota and Iowa; in fact, specimens are so numerous 

 that one particular bed has been designated the Rhinidictya bed. 

 The large amount of material studied by Ulrich caused him to recog- 

 nize in addition to the typical form three varieties, minor, major, and 

 senilis. In his final work upon the subject, variety minor was re- 

 duced to synonymy under the type form of the species on the grounds 



