332 



MOLLUSCOIDEA— BEYOZOA 



PHYLUM V 



tliin inflected walls, tlie apertures irregularly jietaloid. Mesopores numerous, frequently 

 isolating the zocecia, largely filled by a secondary deposit. Acantliopores small and 

 very numerous. Ordovician and Silurian, 



Fic. 475. 



Atactaporella typicalis Ulr. Black River Group ; Minnesota. Surface (A), tangential (7;),, 

 and vertical (' ") sections, i-»/i (after Ulrieli). 



Peronopora Nich. Similar to the preceding but zoaria bifoliate, and zooecial walls 



thicker, not inflected by the acan- 

 tliopores, and more ring -like in 

 transverse section. Ordovician 

 and Silurian. 



Homotrypa Ulr. (Figs. 476, 

 477). Generally ramose, some- 

 times frondescent. Zooecial tubes 

 with very thin and finely crenu- 

 lated walls, and remote diaphragms 

 in the axial region. Cystiphragms, 

 isolated or in series, developed in 

 peripheral region only. Apertures 



structure of walls and parenchymal cord in (A), Homotrypa 

 callosa Ulr., 35/j ; (£), ^tkioporella fro7iilifem Ulr., as/j'; and (C), 

 Retepora columni/era Busk. Recent, 60/i. 



A 



B 



polygonal or sub - circular. Mesopores 

 usually few and restricted to the maculae. 

 Acantliopores generally present. Ordovician 

 and Silurian. 



Homotrypella Ulr. Like Homotrypa but 

 mesopores numerous and cystiphragms 

 usually confined to the early part of the 

 mature region. Ordovician and Silurian. 



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

 Zoarium massive, free. Zoojcial tubes 

 prismatic or cylindrical, thin - walled, 

 A B 



Fio. 477. 



Homotrypa subramo.m Ulr. Black River ; Minnesota. 

 A, Surface. B, Tangential section. i\ Vertical sec- 

 tion, iJ/i. D, H. xp.jKU-ata Ulr. Tangential section, 

 X 17. 



Fin. 478. 



I'rasopora simulatrix Ulr. Trenton ; Ken- 

 tucky. .1, Transverse, and B, Vertical section, 

 i-i/i (after Ulricli). 



