216 CARBONIFEROUS CEPHALOPODA OF IRELAND. 



jPawt/Vy— Rhineceeatid.e, Hi/atf, 1893 (emend. 1900). 



Whorls stout, tetragonal in section, with convex periphery ; longitudinal ridges 

 are developed ; annular lobe present in most genera. 



Genus Tiikincoceeas {Tlijatt, 1893). — Shell thick-discoid, tj-pically large. Cross- 

 section subhexagonal. Periphery broad and dejjressed. Test covered Avith close- 

 set spiral ridges. 



Carboniferous. 



64. Thriucoceras Hyatti.* 65. Tliriucoceras Hiljeruicum. 



Genus Discitocehas {Hyatt, 1883, emend. 1893). — Shell compressed-discoid. 

 Sides and periphery flattened. Outline of aperture distinctly sigmoid. Longitudinal 

 ridges present in the young shell. 



Carboniferous. 



66. Discitoceras Leveilleauum. 68. Discitoceras costellatum. 



67. .. Wriglitii.* 69. .. ? discors. 



Genus Puacocekas {Hyatt, 1883, emend. 1893). — Shell typically discoid, much 

 compressed. Cross-section narrowly sagittate. Periphery acutely angular. Young 

 having apparently the form and proportions of Dlscifuceras at that stage. 



Carboniferous. 



70. Pliacoceras oxystomum. 71. Phacoceras ? rectisuturale.* 



Family— ^OLE^ocnmLiBiE, Hyatt, 1893 (emend. 1900). 



Whorls may be open or closed ; depressed elliptical or broadly hemispherical in 

 outline ; umbilical zone very broad ; shell smooth ; some sj)ecies have spinose, 

 others wing-like projections at the margin of the aperture proceeduig from the 

 umbilical rim. 



Genus AiroCERAS {Hyatt, 1883, emend. 1893). — Shell rapidly increasing, with 

 an open coil. Cross-section sul^triangular. Whorls compressed at the sides, 

 flattened on the dorsal aspect. Test smooth. 



Carboniferous. 



72. Aipoceras comprossiim. 73. Aipocoras Y Haiuusiamim.* 



Genus Ar'AXTiioxAUTiLUs {Foord, 1896). — Shell iV((»^/7».s-like, subglobose, with 

 somewhat flattened peripheral area. Umbilicus rather large, fimuel-shaped, with a 



