BRYOZOA. 



131 



ahlnMlctya.] 



of the branches ; in others, however, those forming the marginal row on each side 

 may be turned slightly outward. Interspaces comparatively thick, less ridge- 

 shaped than usual, often slightly zigzag, with the range of granules well developed. 



Internal structure chiefly diagnostic in vertical sections. These show that the 

 primitive or prostrate cell is comparatively elongate, and that at the turn into the 

 " vestibule " the wall is merely sharply curved and not angular, as in R. mutabilis. 



Associated with this species is a larger form, agreeing in all other respects quite 

 closely with it. At first I thought it identical with R. mutabilis, "and so figured it in 

 1890 (111. Geol. Surv. Repts., vol. viii, p. 304, fig. 2, d, f, and g). At present I should 

 prefer regarding it as a variety of R. neylecta. For the Canadian variety of this 

 species see remarks under R. paupera. 



Compared with other species, R. nicholsoni will be found to have grown differ- 

 ently, the bifurcation of the branches being much less frequent ; the zorecial aper- 

 tures are also more oblique, and vertical sections quite different.. R. mutabilis has 

 wider branches, more direct zoreoial apertures, and different vertical section. 



Formation and locality. Not uncommon in strata equivalent to the Galena limestone of the North- 

 west, at Frankfort, Kentucky, and several localities in Boyle and Mercer counties of that state. Also in 

 rocks of the same age at Nashville, Tennessee. Two fragments supposed to be identical with these 

 Kentucky and Tennessee specimens were collected at St. Paul from the upper shales. 



Mus. Reg. No. 8)04. 



RHINIDICTYA EXIGUA Ulrich. 



PLATE VIII, FIGS. 6-10. 



Rhinidictya exigua ULRICII. 1890. Jour. Gin. Soc. Nat. Hist., vol. xii, p. 184, flg. 9. 



Zoarium bifoliate, small, growing from an expanded basal attachment. Lower 

 portion of branches subcylindrical, with the zooecial apertures here largely filled with 

 a smooth solid deposit of sclerenchyma. Above the first bifurcation the branches 

 have become acutely elliptical in cross-section, their width varying from 0.5 mm. to 

 1.2 mm., with parallel margins, the edges sharp, but in no case seeming to have 

 more than just an appreciable non-celluliferous border. Zooscia in from three to 

 seven rows on each face, their apertures, in the usual state of preservation, appear- 

 ing as impressed, nearly direct, subelliptical or subquadrate, those in the central 

 rows 0.2 mm. long by 0.1 mm. wide, those in the marginal row on each side of the 

 branch sometimes a little larger and often dii-ected somewhat obliquely outward ; 

 all regularly arranged longitudinally, seventeen or eighteen in 5 mm., and separated 

 from each other by rather thin, seemingly smooth interspaces, the latter forming 

 slightly elevated longitudinal ridges. In the specimens originally described and 

 figured, the apertures are somewhat obscured by remains of the shaly matrix, but 

 with several fragments lately discovered among my material from Fountain, Minn.. 



