214 CARBONIFEROUS CEPHALOPODA OF IRELAND. 



often with more or less prominent ridges. Siphuncle subcentral in the direction of 

 the periphery to nearly marginal. 

 Carboniferous. 



G-EOUP OF CcELONAUTILUS PLANOTERGATUS. 



40. Coelonautilus planotergatus. 41. Ccelonautilus Doohylensis.* 



42. Coelonautilus gradus.* 



Genus Steoboceeas (Hyatt, 1883, emend. 1803). — Shell compressed-discoid, with 

 elevated peripheral area and prominent ridges separated by furrows on the sides. 

 Carboniferous. 



Geoup op Steoboceeas sulcatum. 



43. Stroboceras sulcatum. 44. Stroboceras crassum.* 



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

 Cross-section subhexagonal. Peripheral area very narrow, sulcated more or less 

 deeply. Umbilical vacuity large ; the first whorl generally completed before the 

 succeeding one touches it. Young shell with lateral and ventral longitudinal ridges. 



Carboniferous. 



Geoup of Apheleceeas mutabile. 



45. Apheleceras mutabile. 46. Apheleceras Hiberuicum. 47. Apheleceras trochlea. 



Genus Mesochasmoceras (Foord, 1900). — Shell discoidal, much compressed. 

 Whorls slowly tapering, narrow, with a very large umbilical vacuity. Cross-section 

 subhexagonal. Periphery slightly channelled. No ornamentation at any stage of 

 growth. 



Carboniferous. 



48. Mesochasmoceras latidorsatum. 



Genus Diorugoceeas (II yatt, 1893). — Shell with more compressed whorls than 

 those of Apheleceras. Periphery channelled. Involution almost complete in the 

 adult. (This genus is represented by only one species, and is not satisfactorily 

 defined.) 



Carboniferous. 



49. Diorugoceras planidorsatum. 



