New Lower Silurian Bryozoa. 



189 



teen or seventeen in 5 mm. lengthwise and four in 1 mm. 

 obliquely. Apertures elliptical, 0.2 mm. long, half that wide, 

 impressed, the sloping area narrow for this genus, and appearing 

 sometimes a little oblique because of a slight elevation of the 

 posterior border : those in the marginal rows are directed slightly 

 outward. Between the ends of succeeding zocecial apertures one 

 or two small mesopores. There is usually a row of these small 

 pores along the border of the branches. Interspaces narrowly 

 rounded or ridge-shaped, comparatively thin. 



This is a handsome and easily recognized species, 6". interstincta, 

 Ulr. . from the Utica horizon of the Cincinnati group, and the 

 only species of the genus with which it need be compared, has 

 more numerous mesopores, wider branches, and much wider slop- 

 ing areas about the smaller zooecial apertures. I have described 

 three other species of this genus from the Trenton Shales of 

 Minnesota, and there is yet another that remains undescribed. 

 S. angularis has wider branches, has numerous mesopores and 

 much thicker ridge-shaped interspaces between the zooecial 

 apertures. S. frondifera grows into broad fronds and has zooecia 

 like those of S. angularis. S. cribrosa forms cribrose zoaria 

 resembling those of Clathropora. 



Position and locality: Rare in the Trenton Shales at Fountain, 

 Minn. 



ARTHROSTVLUS CONJUNCTUS, n. Sp. 



<60 



Fig. 14.— Arthrostylus conjunctus, n. sp., and A. obliquus, n. sp. a, lateral view of 

 central portion of segment of A. conjunctus xl8. b, view of portion of non-cellulifer- 

 ous side of same, c, lateral view of central portion of segment of A. obliquus, xl8, 

 d, non-celluliferous side of same, showing about three-fourths of the segment with 

 the upper extremity, xl8. e, transverse section of Arthrostylus tenuis, James, sp., x50. 

 /, obverse side of upper end of segment of same, magnified twenty-eight diameters. 



