662 PALEONTOLOGY OF ILLINOIS. 



placed by Hall under his Orthopora,, as also several of my 

 species of Rhombopora should, perhaps, have been referred 

 here. 



ACANTHOCLEMA CONFLUENS Ulrich. 

 PI. LXX, Fig. 5-5b. 



Zoarium a slender stem, 1 mm. or a little more in diameter, 

 bifurcating at intervals, frequently found flattened by pressure 

 in the shales. Zooecia apertures oval, 0.12 mm. long, two-thirds 

 as wide, arranged very regularly in longitudinal, transverse, and 

 diagonally ascending series, between sharply angular, and regu- 

 larly fiexuous, longitudinal ridges, which alternately recede and 

 approach each other, without, however, at any time coming in 

 contact. The sloping areas are confluent, 7. e., they communi- 

 cate with each other by means of a narrow channel which is 

 left between the ridges. Between this channel and the lower end 

 of the aperture there is a distinct, though not large tubercle, 

 that scarcely projects beyond the level of the ridge summits. 

 Measuring longitudinally, between nine and ten apertures occur 

 in 4 mm.; diagonally, about nine in 2 mm.; transversely, five 

 in 1 mm. 



None of the species known to me resemble this one sufficiently 

 to require comparisons. 



Position and locality: Keokuk group, Nauvoo, 111. 



BACTROPORA Hall, 1887. 



(Pal. N. Y. Vol. VI, p. XV.) 

 (For generic diagnosis see page 402.) 



Species of this genus are characterized by a free and un- 

 branched zoarium, the basal extremity being pointed and the 

 remainder a simple, slightly curved or straight, solid, cylindri- 

 cal stem. In their minute features the zooecia and zoarium 

 differ in no essential respect from Rhombopora. The "solid" 

 character of the stems distinguishes them from Cceloconus. 



