648 PALAEONTOLOGY OF ILLINOIS, 



marked by longitudinal ridges and a strong spine which is situ- 

 ated on the depressed space between the ends of the zooecia 

 apertures. ' A. confluens, of the present work, has these two 

 essential features, but I very much fear that the division will 

 prove of little value, as it will be exceedingly difficult to draw 

 the line between it and Rhombopora. A second group em- 

 braced by Hall's Acanthoclema are widely different from his 

 type species, and belong to my genus Streblotrypa. Of these 

 I may mention A. scutvlatum Hall, and A. hamiltonense Nich- 

 olson, sp. 



RHOMBOPORA SUBANNULATA Ulrich. 



PI. XLV, flg. 1-li. 



Zoarium dendroidal, branching dichotomously at intervals of 

 15 mm. or more, and inclining to spread in the same plane. 

 Branches from 1.2 to 2 mm. in diameter, marked with more 

 or less noticeable annulations between 1.5 and 2 mm. apart. 

 Zooecia proceed obliquely out from the axis of the branch at 

 an angle of about 45, a,nd have irregularly, flexuous thin 

 walls. Two or three superior hemisepta and an occasional in- 

 ferior hemiseptum are seen at the close of the immature region 

 in vertical sections. The zooecia walls are in contact. In young 

 examples they are comparatively thin, the apertures sub-rhom- 

 boidal in shape and arranged in diagonal intersecting series 

 winding spirally around the branch. Measuring diagonally ten 

 in 2 mm. At each angle a small but distinct tubercle. As the 

 zoarium matures, the apertures become smaller and subcircular 

 or oval in shape, the interspaces thicker, multigranose, and 

 occasionally form faint longitudinal ridges. Now there are 

 nine apertures in 2 mm. diagonally. 



In transverse and tangential sections the strong hemisepta 

 cause appearances likely to be mistaken for diaphragms. 



This species resembles several others but is clearly distinct. 

 The superficial appearance, as shown in the figures, varies 

 greatly according to the age of the examples. With the aid of 

 tangential sections the extremes are brought together, showing 

 what may also be seen by examining the opposite ends of large 

 fragments, that they are really due to age. E. varians, of the 

 Keokuk group, shows similar variations. 



Positition and locality: Hamilton group, Buffalo, Iowa. 



