332 



MOLLUSCOIDEA— BRYOZOA 



PHYLÜM V 



thin inflected walk, the apertiires irregularly petaloid. Mesopores niimerous, frequently 

 isolating the zooecia, largely filled by a secondary deposit. Acanthopores small and 

 very niimerous. Ordovician and Silurian. 



c 



Fio. 475. 



Atactaporella typicalis Ulr. Black River Group ; Minnesota. Surface (.1), tangential (11), 

 and vertical (C) sections, ^% (after Ulrich). 



Peronopora Nicli. 



A 



Similar to the preceding but zoaria bifoliate, and zoa3cial walls 

 B c thicker, not inflected by the acan- 



thopores, and more ring-like in 

 transverse section. Ordovician 

 and Silurian. 



Homotrypa Ulr. (Figs. 476, 



477). Generally raniose, some- 



times frondescent. Zooecial tiibes 



with very thin and finely crenii- 



lated walls, and remote diaphragms 



in the axial region. Cystiphragms, 



isolated or in series, developed in 



peripheral region only. Apertures 



polygonal or sub - circiilar. Mesopores 



iisually few and restricted to the maciilae. 



Acanthopores generally present. Ordovician 



and Silurian. 



Homotrypella Ulr. Like Homotrypa but 

 mesopores niimerous and cystiphragms 

 usually confined to the early part of the 

 mature region. Ordovician and Silurian. 

 Prasopora Nich. and Eth. (Fig. 478). 

 Zoariiim massive, free. Zooecial tiibes 

 prismatic or cylindrical, thin - walled, 



A B 



Structure of walls and parenchymal cord in {A), Homotrypa 

 callosa Ulr., 35/j ; (B), Stictoporella frondifera Ulr., »ö/^ ; and (C), 

 Retepora columnifera Busk. Recent, 60/^. 



Fig. 477. 



Homotrypa subramosa Ulr. Black River ; Minnesota. 

 A, Surface. ' B, Tangential section. C, Vertical sec- 

 tion, 14/j. X», H. sejxtrata Ulr. Tangential section, 

 X 17. 



Fx(i. 478. 



Prasopora simulatrix Ulr. Trenton ; Ken- 

 tucky. A, Transverse, and B, Vertical section, 

 i4/i (after Ulrich). 



