396 PALAEONTOLOGY OF ILLINOIS. 



I'NITRYPA Hall. Zoaria infundibuliform; zocpcia in two ranges. 

 Keels as in Semicoscinium, but with their summits connected 

 by sub-imbricating transverse bars or plates, which may be as 

 numerous as the zooecia, or only two to each fenestrule. 



Type: U. lata, Hall. Range, Lower Helderberg to Hamilton. 



HEMITRYPA 1'hillips. Zoaria funnel-shaped or undulating- 

 foliar expansions; branches rigid. Zocecia in two ranges, their 

 apertures separated by a moderately developed keel. The latter 

 is elevated at regular intervals into small pillars, which, when 

 the superstructure they support is worn away, appear as spine- 

 like prominences. The superstructure consists of straight or 

 zigzag longitudinal bars, of which one is placed over each 

 branch upon the row of pillars, and another usually somewhat 

 thinner, suspended midway between the branches. These bars 

 are then connected by transverse processes, so as to leave regu- 

 lar, small, generally hexagonal openings, corresponding in num- 

 ber and position Avith the zooecial apertures beneath them. 



Type: H. oculata Phillips. Range, Clinton to St. Louis. 



HELICOPORA Claypole. Zoaria spiral, the inner edge thickened 

 and non-poriferous, without, however, forming a solid central 

 axis. Other characters as in Fenestella. 



Type: H. latispiralis Claypole. Range, Niagara and Devonian. 



ARCHIMEDES Lesueur. Zoaria like those of Helicopora, but 

 differing in having a solid central axis. 

 Type: A. wortheni Hall. Range, Keokuk to Chester. 



LYROPORA Hall. Zoaria flabellate, the fenestrated portion 

 spread between two strong, non-celluliferous. diverging supports. 

 Zooecia in from two to five rows. Median keel obsolete. 



Type: L. quincuncialis Hall. Range, Burlington to Chester. 



FENESTRALIA Prout. Zoaria strong, with two rows of cells on 

 each side of the median keel. In other respects like Polypom. 



Type and only known species: F. sancti-ludovici Prout. St. 

 Louis group. 



POLYPORA McCoy. Zoaria in most respects like Fenestellfi, 

 but differing in having from two to six or even eight rows of 

 cells, and in wanting the characteristic median keel. The latter 



