New Lower Silurian Bryozoa. 195 



Zoarium jointed, so far as known consisting of primary and 

 secondary segments only. Primary segments, from 3 to 5 mm. 

 long, 0.5 to 1.0 mm. in diameter, the thickest generally the 

 longest, and always with a well-marked socket for articulation with 

 the first of the secondary segments. Only one socket on any seg- 

 ment, and some of them are without any. The socket is situated 

 generally a little above the center of the length. Some of the 

 joints of this set are decidedly compressed or irregularly shaped, 

 and all more or less distinctly striated longitudinally. Both ends 

 truncate, but with the central portion of the upper convex, and 

 the lower more acutely drawn out. Zocecia usually in six ranges, 

 their apertures oval, nearly direct, o. 1 mm. long, with about 

 seven in each range in 2.5 mm.; a strong tubercle just behind or 

 near each aperture. 



Secondary segments of about the same length as those of the 

 primary set. They are, however, more slender, none being more 

 than 1.5 mm. in diameter, and the smallest ones only about 0.3 

 mm. Upper end terminating abruptly, spinous, lower end 

 rounded. Zooecia is six and sometimes seven, rows arranged 

 between longitudinal ridges that become stronger with age; a trans- 

 verse arrangement also usually prevails; apertures oblique, with 

 the inferior border very prominent, spine-like, seven in 2.5 mm. 



Mature and well-preserved segments of this species could not 

 well be confounded with any other jointed bryozoan known to me. 

 Immature joints of the secondary series must be examined with 

 some care, to separate them from the segments of the associated 

 Helopora mucronata. In that species the segment enlarges gradu- 

 ally from the pointed inferior end, the zooecial apertures are not 

 arranged between longitudinal ridges, and above each aperture 

 there are two thin striae, which are wanting in this species. The 

 spine-like prominence of the inferior margin of the zooecial 

 apertures serves sufficiently in distinguishing A. armatwn from the 

 other species of the genus. 



Position and locality: Top of the Trenton Shales, at Cannon 

 Falls, Minn., where it is associated with numerous other small 

 bryozoa. 



