224 THE PALEONTOLOGY OF MINNESOTA. 



[Atactbporella insueta. 



zoarium. On plate XV, fig. 16 illustrates the characters as brought out in a section 

 taken from a young example. The section drawn on place XVIII, fig. 2, was pre- 

 pared from the thickest specimen seen. Between these two figures the student will 

 be able to work out the essential features without further comment. It might be 

 well, however, to direct attention in the last figure to the infilling and contraction 

 of the mesopores in the upper half. Also to the fact that some of the zooecial tubes 

 in vertical sections, and the same remark applies to all Bryozoa having cystiphragms, 

 may appear to be, in part at least, crossed by complete diaphragms. This appearance 

 however, is merely the result of the different directions in which the cystiphragms 

 cross the various tubes shown in the section. To obtain the characteristically curved 

 line of the cystiphragm it is necessary that the section pass nearly across them. 

 When the section passes through the tube parallel with the inner edges of the cysti- 

 phragms they must necessarily appear as straight or oblique lines, thus simulating 

 true diaphragms. 



This variety differs from the typical form of the species in the following particu- 

 lars : The zooecia are a little larger, the mesopores less numerous and often of larger 

 size, the acanthopores one or two more to each zooecium, and the tabulation a little 

 more compact. Compared with other species, A. typicalis is readily distinguished by 

 the exceeding tenuity of the zooecial walls, and the greater projection inward of the 

 acanthopores. The latter are more numerous than in any of the other Minnesota 

 species known. 



Formation and locality. The types of A. typicalis are from the Utica horizon of the Cincinnati sec- 

 tion at Cincinnati, Ohio, and vicinity. The present variety prcecipta is from the middle third of the 

 Trenton shales at Minneapolis, St. Paul, Fountain, and other localities in Minnesota. 



Mus. Beg. Nos. 5983, 5984. 



ATACTOPORELLA INSUETA, n. sp. 



PLATE XV, FIGS. 13-15; PLATE XVIII. PIGS 5-8. 



Zoarium forming thin crusts over shells, crinoid columns, and ramose Bryozoa, 







generally about 1 mm. in thickness. Surface with clusters of large cells at intervals 

 of 3.3 mm., usually little or not at all elevated, at other times rising into low and 

 broad monticules. Zocecial apertures in old stages subangular and with thin inter- 

 spaces in which the mesopores are not readily distinguishable; in younger stages more 

 rounded, often ovate, with the interspaces usually somewhat wider and the meso- 

 pores obvious ; walls thin, but little inflected. In the commonest form of the species 

 there are from thirteen to fifteen zooecial apertures in 3 mm., but in the variety 

 illustrated in figs. 7 and 8 on plate XVIII, only eleven are to be counted in the same 

 space. Acanthopores of medium size, two to four to each zooecium, situated in the 



