BRYOZOA. 663 



BACTROPORA SIMPLEX Ulrich. 



PI. LXX, flg. 14-14b and PI. LXXI, fig. 6-6a. 



Zoarium free, consisting of a simple, unbranched, straight or 

 slightly curved stem. An average specimen is 1.5 mm. in diam- 

 eter and 18 mm. long. The largest seen is nearly 30 mm. long 

 and 1.8 mm. in diameter. The upper extremity is rounded, the 

 lower is striated and tapers rapidly to a point. Surface with 

 small, transversely elongated monticules which often appear as 

 annulations but really never continue more than half way 

 around the stem, and only rarely more than one-third the way. 

 These semi-annulations or monticules are developed in two 

 alternating series on opposite sides of the cylindrical stem. 

 Those of each series are placed directly above one another, the 

 distance from summit to summit being about 1.5 mm. Zocecia 

 apertures oval or sub-circular, about 0.12 mm. in diameter, ar- 

 ranged in spiral diagonal lines which intersect each other at an 

 angle of about 67. Measuring diagonally, nine apertures occur 

 in 2 mm.; transversely, five in 1 mm. Interspaces seldom equal 

 in width to the diameter of the apertures, generally considera- 

 bly less, thickest and less acutely rounded toward the base, the 

 summit occupied by a single or double row of small closely set 

 acanthopores, which have a peculiar rayed appearance in tan- 

 gential sections. 



Zooecial tubes thin-walled in the axial region, proceeding from 

 the center of the stems toward the surface at an angle of about 

 45. At the beginning of the peripheral region their walls are 

 much, though not excessively, thickened. In this zone the 

 peculiar stellate acanthopores make their appearance. In verti- 

 cal sections they appear as vertical series of minute dots. 



The free and unbranched character of the zoarium of this 

 species is so marked a feature that complete specimens may 

 be distinguished at once from associated bryozoa. The trans- 

 versely elongated monticules are also very distinctive. The 

 Devonian types of the genus are without surface elevations. 



Position and locality: Keokuk group. Abundant at Keokuk, 

 Iowa; Nauvoo and Warsaw, 111. 



