ORDER IV 



CRYPTOSTOMATA 



343 



Sceptro^wra Ulr. Segments short, greatly expanded above, celluliferous all aroimd. 

 Ordovician and Silurian. 



U^ 



f.- 7\ 



m 



W-J 



a, Arthroclema bülingsi Ulr. 2/3. ^^ 



Fig. 501. 

 A. armatum Ulr. Segment, 12/j. 



c, Nematojwra conferta Ulr. 6/^, 



d, e, Ärthrostylus conjunctus Ulr. 12/j. /^ Helopora harrisi Ulr. 12/j (after Ulrich). 



Arthroclema Bill. (Fig. 501, a, b). Segments sub-cylindrical, celluliferous on all 

 sides, arranged pinnately. Articulation both terminal and lateral. Ordovician. 



Nematopora Ulr. (Fig. 501, c). Zoaria very slender, 

 ramose, continiious above the pointed basal extremity. Zooecia 

 sub-tubular, arranged radially about one or two minute axial 

 tubes. Ordovician and Silurian. 



Glauconome Goldf. (Penniretepora d'Orb.). Zoarium branch- 

 ing continuously ; reverse side non-celluliferous ; zooecia as in 

 Nematopora. Ordovician and Silurian. 



Family 5. Rhabdomesontidae Vine. 



Zoaria ramose or simple, not articulated, sometimes with a 

 large or small axial tube, and generally solid. In the latter 

 case the axial region is occupied by thin-walled primitive tubes, 

 with or without diaphragms. Hemisepta usually present, but 

 never conspicuous. External zooßcial apertures oval or circular, 

 regularly arranged, and usually at the bottom of a rhombic or 

 hexagonal sloping area, or between longitudinal ridges. Mesopores absent. Ordovician 

 to Permian. 



Rhombopora Meek (Fig. 503, b). Zoaria slender, ramose, solid. Zooccial tubes 

 witb tbe outer or vestibulär region thick-walled, apertures arranged in diagonal 

 or longitudinal lines. Strong acanthopores and smaller spines generally present. 

 Ordovician to Permian. 



Bactropora Hall. Zoaria simple or only slightly brancbed, the lower extremity 

 pointed. Lower Carboniferous-. 



Rhabdomeson Young. Differs from Rhombopora only in having a slender axial 

 tube, to wbich the proximal ends of the zocEcia are attached. Goal Measures and 

 Permian. 



Goeloconus Ulr. (Fig. 503, a). Zoaria simple, hollow, expanding gradually from 

 the striated base ; substance thin. Primitive portion of zooecia short, with well- 

 developed hemisepta. Lower Carboniferous. 



Fig. 502. 

 Helopora spiniformis Ulr. 

 Stones River ; Tennessee. 



A, Vertical section, l^/i. 



B, Segment, l/i and l4/l 

 (aft«r Ulrich). 



