644 PALAEONTOLOGY OF ILLINOIS. 



HELOPORA IMBRICATA Ulrich. 



PL XXIX, Fig. 5. 



Segments with a rough aspect, 3 or 4 mm. in length, and 

 0.5 mm. in diameter; upper end flattened, lower extremity ob- 

 tusely pointed and striated. Zooecia in seven or eight vertical 

 series around the segment, the rows being separated by rather 

 inconspicuous carinae; also in much more conspicuous trans- 

 verse series, on an average 0.3 mm. distant from each other. 

 Apertures when worn, oblong quadrate; in the perfect state ob- 

 lique, sub-circular or ovate, 0.1 mm. wide, with the lower mar- 

 gin strongly elevated and produced posteriorly into three small 

 ridges, the central one terminating at or within the mouth of 

 the preceding cell, is the most persistent, while the other two, 

 which continue into its margin, are generally fainter and may 

 be obsolete. The transvere approximation of the zocecia and 

 the prominence of the posterior margin impart a characteristic 

 imbricating appearance to the segments. 



This species resembles H. spiniformis Ulrich, from the base of 

 the Trenton group of Tenn., but is distinguished by the shorter 

 segments, more pronounced aperture margin, and the ridges 

 between the ends of the cells. 



Position and locality: Cincinnati group. Isolated segments 

 are not uncommon on the surface of limestone slabs at Wil- 

 mington, 111. 



NEMATOPORA Ulrich. 



(For generic diagnosis see page 401.) 



The typical and earliest known species of this well marked 

 and natural group of Palaeozoic Bryozoa is the N. quadrata, 

 n. sp.,* from the Trenton limestone of New York. The next 



* The characters of this species are as follows: Zoarium slender, dichotomously 

 branched at varying intervals; branches quadrangular, 0.3 or 0.4mm. wide. Zooecia 

 arranged alternately in four longitudinal series, separated by a double ridge. Aper- 

 tures oval, 0.4 mm. wide and nearly twice as long, four in 2 mm. lengthwise ; surrounded 

 by distinct peristome ; peristomes connected longitudinally by a thin ridge. (PL XXIX, 

 fig. 12.) 



The double ridge between the ranges of cells is the most conspicuous peculiarity of 

 this species. It is common on slabs from Trenton Falls, N. Y. 



