GENUS CCELONAUTILUS. 213 



Genus Poteriocebas (M'Coy, 1844). — Shell fusiform, the curvature slight, very 

 slender in the apical portion, inflated in the middle, contracting towards the 

 aperture, which is simple. Cross-section nearly circular in the young, elliptical in 

 the adult. Siphuncle subcentral to marginal, moniliform. Body-chamber pro- 

 portionately large, constricted near the aperture. Test smooth. 



Ordovician to Carboniferous. 



35. Poterioceras fusiforme. 36. Poterioceras latiseptalum. 



37. Poterioceras ventricoaum. 



Family— Tainoceratim;, Hyatt, 1883 (emend. 1893, 1900). 



Discoidal, whorls usually robust, trapezoidal in section at some stage of growth, 

 or throughout life, tuberculated ; sutures without annular lobe ; siphuncle small 

 and cylindrical. 



Genus Temnocheilus (M'Coy, 184-1). — Shell subdiscoidal, umbilicus generally 

 large and deep. Periphery broad and flattened. Whorls usually but little 

 embracing ; cross-section trapezoidal at all stages of growth. One row of nodes 

 on each side of the periphero-lateral margin. 



Carboniferous. 



38. Temnocheilus coronatus. 



Family — Teigonoceratid.e, Hyatt, 1883 (emend. 1893, 1900). 



Whorls subtriangular at some stage of growth or throughout life, often with 

 longitudinal ridges; body-chamber sometimes free near the aperture. Annular 

 lobe rarely present. Siphuncle small, submarginal. 



Genus Trigonoceras (M'Coy, 1844). — Shell rapidly tapering, the apical part 

 forming half a volution (fide M'Coy), the rest gently curving. Cross-section scuti- 

 lorm. Peripheral area distinctly concave; anti-peripheral a\^i' carinated. Siphuncle 

 subcentral in the direction of the peripheral area. Obtuse longitudinal ridges occur 

 at- the peripheral angles. 



Carboniferous. 



:!!•. Trigonoceras paradoxicum. 



Genus Ooelonautilus (Foord, L891, emend. Hyatt, L893). — Shell compressed - 

 discoid, deeply umbilicated. Cross-section subtriangular. Peripheral area very broad, 



