Ulrich on Genera of 'Bryozoa. 
233 
ranged in a bl-pinnate manner, those forming the central stem 
being the largest. Each of the primary segments has one or 
two sockets on each side for articulation with a smaller second- 
Fig. 2. Thin sections illustrating the internal structure of Helopora 
fragilis Hall, from the Clinton group at Hamilton, Ontario, and Helofora 
lindstromi n. sp.* from the Upper Silurian of Gotland. 
All the figures are magnified 18 times. </, vertical section of small 
segment of H. fragilis^ showing the zooecia as they diverge from the 
central axis. ^, small portion of a tangential section of the same, showing 
acanthopores at the surface, c, tangential section of the same, as it ap- 
pears in the ferruginous matrix; in these examples the extreme outer 
region is destroyed; this section, however, still shows good indications of 
acanthopores and, near the center of the figure, a close set row of much 
smaller pores, rf, transverse section of the same passing through seg- 
ment near its upper extremity. «, transverse section of the same cutting 
the segment just above its basal extremity. /", tangential section of H. 
lindstromi^ showing form and arrangement of zooecia and acanthopores. 
g^ transverse section of the same. //, vertical section of the same. 
1 The cylindrical or slightly club-shaped segments of this fine species 
vary in length from loto 15 mm., and in diameter from 1.3 to 1.8 mm. Both 
extremities are without cells and nearly smooth, the upper one flattened 
