BRYOZOA. 645 



species occurs in the Alexander Co. beds of the Cincinnati group. 

 (The exact age of these beds is somewhat doubtful, and I am 

 inclined to regard them as more likely representing an upper 

 member of the Trenton group). Four species are known and 

 now described from the horizon. So far as known, the genus is 

 represented only by an undescribed species in undoubted Cin- 

 cinnati group deposits, but from the divisions of the Anticosti 

 group, Billings describes five forms under Helopora that have 

 the characteristic features of the genus. These are H. concava, 

 forwosa, Hneata, striatopora. and strigosa. The H. lineopora 

 may also belong here. Of Niagara species, Hall's Trematopora 

 mJnuta and one or two undescribed species are congeneric with 

 JV. quadrat a. As yet the genus is unknown above the Niagara 

 formation. 



NEMATOPORA RETRORSA Ulrich. 



PI. XXIX, fig. 9-9b. 



Zoarium ramose, sub-cylindrical, branching irregularly, about 

 0.6 mm. in diameter. Zocecia arranged in longitudinal series, 

 about 5 in two mm., between eight to ten straight and moder- 

 ately prominent ridges. Apertures sub-circular. 0.1 mm. in 

 diameter, slightly truncated, and sloping on the posterior side; 

 the anterior margin direct, sharp, sometimes with a perceptible 

 rim. Immediately below the orifice the interspace exhibits three 

 faint striae. Walls of zocecia very thin, appearing somewhat 

 sickle-shaped in vertical section. 



The backward direction and the subcircular form of the zoce- 

 cial apertures are characteristic features. In A 7 , fragilis the 

 apertures are also directed backwards, but they differ in being 

 oval, while the longitudinal ridges are less distinct or absent, 

 and the rows of zooecia only six, instead of eight to ten. 



Position and locality: Base of Cincinnati group. Alexander 

 County, Illinois. 



