344 



MOLLÜSCOIDEA— BEYOZOA 



PHYLUM V 



Nemataxis Hall. 



a 



Devoiiian. Nematotrypa Bassler. Ordoviciaii. Orthopora Hall. 



f 



Fig. 603. 



a, Coeloconus rliomhicu» Ulr. Vi and surface i^/i. h, Rhoinbopom incrassata Ulr. i/^ and ]«/i. c, StreUotrypa 

 major Ulr. i/j and 18/^. d, Acanthodema conflucns Ulr. i/i and 24/j (after Ulrich). 



Silurian and Devonian. Hyphasmopora Etlieridge. Carboniteroiis. Acanthodema 

 Hall (Fig. 503, d). Silurian to Lower Carboniferous. 



Tropidopora Hall. Devonian. Strehlotrypa Ulricli (Fig. 503, c). Devonian and 

 Lower Carboniferous. 



Family 6. Ptilodictyonidae Ulrich. 



Zoaria hifoliate, composed of two layers of zomcia grown together back to back, usualhj 

 joined at hast at the base, and forming leaf-like expansions, or compressed, branching 

 or inosculating stems. Mesial plates ivühout median tubuli ; hemisepta usually present. 

 Inner orifice generally semi-elliptical, the outer more rounded, usually ovate, and 

 surrounded by either a sloping area or a ring-like peristome. Vestibules separated by 

 thick walls. Ordovician to Devonian. 



Ptilodictya Lonsd. (Heterodictya Nich.). Zoaria lanceolate or falciform, witli a 

 small basal expansion. In tlie young condition the zoariuni consists of longitudinally 

 arranged, narrow, oblong-quadrate zocecia, new zooccia, of different width and arrange- 





Fig. 504. 



a, Esckaropora angularis Ulr. Vi and surface s/j. h, E. subrecta Ulr. »/j. c, d, Stirtoporella cribrosa Ulr. Vi 

 and surface i8/i. Black River of Minnesota (after Ulrich). 



ment, being added subsequently on each side. In the vestibulär or outer region the 

 walls are more or less thickened, solid, and with a double row of exceedingly minute 

 dots. Silurian and Devonian. 



