192 Cincinnati Society of Natural History. 



HELOPORA ALTERNATA, n. Sp. Fig. 16,/. 



Zoarium jointed ; Segments exceedingly slender, slightly curved,, 

 about 3.0 mm. long, and 0.5 mm. in diameter; lower extremity 

 obtusely pointed. Zocecia apertures oval, direct, impressed, com- 

 paratively large, 0.14 mm. long, arranged alternately, four in each 

 cycle, twelve cycles in 2.5 mm. Interspaces rather thin, generally 

 appearing simply rounded. On the segment illustrated two faint 

 impressed lines wind between the apertures near the base. 



The very thin segments, and large alternately or quincuncially 

 arranged zocecia impart to this species a very characteristic and 

 easily recognized appearance. 



Position and locality: Base of Trenton Shales, Minneapolis,. 

 Minn. 



HELOPERA MUCRONATA, n. Sp. Fig. 1 6, e. 



Zoarium jointed. Segments spine-like, slightly curved, 3.5 to 

 4.0 mm. long, tapering downward from a diameter of 0.6 or 0.7 

 mm. to the acute and finely striated basal extremity. Zocecia 

 apertures oblique, the inferior and lateral margins elevated, 

 arranged longitudinally and spirally with about seven in one volu- 

 tion, and six in 2.5 mm. lengthwise. At the upper side of each 

 aperture there are two short striae, which become obsolete as they 

 pass into the cell mouth. 



The curved and tapering form, acute, striated, lower extremity, 

 and spiral arrangement of the zocecial apertures distinguish this 

 species from all others of the genus known. 



Position and locality : Top of Trenton Shales, Cannon Falls, 

 Minn. 



ARTHROCLEMA, BilHngS. 



• / 



^^^-^Kiiiw 



Fig. n.—Arthroclema Billingsi, Ulrich. A nearly complete example of this species r 

 from the Trenton limestone at Ottawa, Canada, introduced to illustrate the mode of 

 growth prevailing in this genus. The species is described in Vol. VIII. , Reports 

 Geological Survey of Illinois, p. 642 (in press) (Note.— This figure is turned side- 

 ways to adjust it to the length of the page.— Ed.) 



