642 PALAEONTOLOGY OF ILLINOIS. 



number in the length of a segment varying from four to seven, 

 generally five or six. One specimen which I collected at another 

 locality* is clearly distinct from Billings' species. This species I 

 propose to call A. billings!, in honor of the eminent founder of 

 the genus. It is distinguished as follows: 



The primary segments are 3.5 to 4 mm. in length, and 0.8 

 mm. in diameter. The first of each of the secondary segments 

 is also about 4 mm. in length, while the remaining secondary, 

 and all the tertiary joints, are only 2 mm. long by about 0.5 

 mm. in diameter. The most striking peculiarity is that each of 

 the primary segments articulated with four secondary segments, 

 two upon each side. Compared with A. pulchellum the cell 

 apertures are more crowded and thin-walled, as well as sub- 

 quadrate instead of oval, though also six in two mm. The 

 whole zoarium also has a more rigid appearance than has been 

 observed in any example of that species. (PL XXIX, fig. 6c.) 



HELOPORA Hall, 1852. 



(Pal. N. Y. Vol. II, p. 44.) 

 (For generic diagnosis see page 401.) 



The typical species of this genus is Hall's H. fragilis, a very 

 abundant and characteristic fossil of the Clinton group of Can- 

 ada. Beside the type species, seven others are known to me 

 that are constructed upon the same general plan,t four of them 

 Lower Silurian and three Middle or Upper Silurian. The Lower 

 Silurian species differ from the Upper Sifurian and typical 

 section of the genus, in having the cell apertures arranged in 

 longitudinal series between elevated ridges, and the interspaces 

 between the ends of the apertures longer. In H. fragilis the 

 zocecia apertures are oval or sub-quadrate, with rather thin 

 equal walls. In H. Undstromi, n. sp., they are ovate, with hexa- 

 gonal margins. The Lower Silurian species are also without 

 the small acanthopores which are found in the Upper Silurian 



*In the city of Ottawa, at the base of the bluffs near the ferry landing. 



* In the Catalogue of Silurian Fossils of Anticosti, Mr. E. Billings describes twelve 

 species, which he refers to Helopora. Upon examination only three of these prove con- 

 generic with H. fragilis. 



