216 CARBONIFEROUS CEPHALOPODA OF IRELAND. 



Family — Rhineceeatid^;, Hyatt, 1893 (emend. 1900). 



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

 are developed ; annular lobe present in most genera. 



Genus Theincoceras {Hyatt, 1893). — Shell thick-discoid, typically large. Cross- 

 section subhexagonal. Periphery broad and depressed. Test covered with close- 

 set spiral ridges. 



Carboniferous. 



64. Thrincoceras Hyatti.* 65. Thrincoceras Hibernicum. 



Genus Discitoceeas (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 Leveilleauuiu. 68. Discitoceras costellatum. 



67. ,, Wrigkfcii.* 69. „ ? discors. 



Genus Phacooeeas (Hyatt, 1883, emend. 1893). — Shell typically discoid, much 

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

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



Carboniferous. 



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



Family — Solenocheilid,e, Hyatt, 1893 (emend. 1900). 



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

 outline; umbilical zone very broad; shell smooth; some species have spinose, 

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

 umbilical rim. 



Genus Aipoceras (Hyatt, 1883, emend. 1893). — Shell rapidly increasing, with 

 an open coil. Cross-section subtriangular. Whorls compressed at the sides, 

 flattened on the dorsal aspect. Test smooth. 



Carboniferous. 



72. Aipoceras compressum. 73. Aipoceras ? Haiiiesianuin.* 



Genus Acanthonatjtilus (Foord, 1896). — Shell Nautilus-\ike, subglobose, with 

 sonic what flattened peripheral area. Umbilicus rather large, funnel-shaped, with a 



