370 PAL/EONTOLOGY OF ILLINOIS. 



occasionally wanting, in other cases numerous; when present 

 they are angular and intersected by crowded horizontal dia- 

 phragms, often obscured by a secondary deposit. Acanthopores 

 abundant. Cystiphragms always present in the mature region. 



MONTICULIPORA d'Orbigny. Zoaria massive, lobate or lamel- 

 late, incrusting or free. Surface with monticules, or even. 

 Zooecia polygonal, thin-walled. Mesopores very few, generally 

 entirely absent. Acanthopores more or less numerous. 



Type: M. mammulata d'Orb. Kange, Trenton to Hamilton. 



ATACTOPORELLA Ulrich. Zoaria generally forming thin crusts 

 over foreign bodies, rarely lobate, or subramose. Surface with 

 monticules. Zooecia with very thin inflected walls, the apertures 

 irregularly petaloid. Mesopores numerous, frequently isolating 

 the zooecia, and largely filled, especially near the surface, by a 

 secondary deposit. Acanthopores very abundant, generally en- 

 croaching upon the zocecial cavity. 



Type: A. typicaJis Ulr. Kange, Trenton to Cincinnati. 



HOMOTRYPELLA Ulrich. Irregularly ramose. Monticules want- 

 ing, small maculae usually present. Zooecia with moderately 

 thick or thin walls. Apertures sub-circular, sometimes faintly 

 petaloid. Mesopores abundant, more or less completely isolat- 

 ing the zooecia. Cystiphragms generally developed only in the 

 median region of the zooecial tubes, being usually absent just 

 beneath the surface and never present in the axial region. 

 Acanthopores numerous. 



Type: H. instabilis Ulr. Range, Trenton to Clinton. 



PERONOPORA Nicholson. Zoaria bifoliate, the surface even, 

 usually with small maculae. Mesopores and aeanthopores vary- 

 ing in number. Cystiphragms abundant, present throughout the 

 mature region in the zooecial tubes. Apertures circular or poly- 

 gonal. Zooecial walls comparatively thick, sometimes ring-like 

 in transverse section. 



Type: P. decipiens Rominger, sp. Cincinnati group. 



HOMOTRYPA Ulrich. Ramose to frondescent, with or without 

 monticules. Mesopores occasionally wanting, always few, some- 

 times gathered into clusters. Zooecial tubes with verv thin walls 

 and remote diaphragms in the large axial or immature region ; 



