THE CEPHALOPODA. 211 



40. Hbrcoceras, Barrande, 1865. 



Turns of the shell at first in contact, slightly overlapping, afterwards spirally 

 divergent ; siphuncle marginal on outer side, edge of exterior chamber contracted and 

 compressed transversely ; Silurian. 



6. Shell neither spiral nor linear. 



41. GoMPHOCERAS, Sowerby, 1839. = Orthoceras {pars), Sowerby, 1812. = 

 Conilites, Pusch, 1837. = Nelimenia, Castelnau, 1843. = Bolboceras, Fischer, 1844. 

 = Apioceras, Fischer, 1844. = Poterioceras, M'Coy, 1844. = Lituites {pars) 

 Quenstedt, 1846. = Syncoceras {pars), Pictet, 1854. 



Shell straight, pear-shaped ; aperture of exterior chamber contracted, small and 

 lobed, the exterior chamber somewhat globular; position of siphuncle variable; from 

 Silurian into Carboniferous. 



42. Phragmoceras, Broderip, 1839. = Campulites {pars), Deshayes, 1830. 

 = Orthoceratites, Steininger, 1831. 



Shell slightly curved, pear-shaped; aperture of exterior chamber contracted, small 

 and lobed ; siphuncle generally on the internal (concave) side ; from Silurian into 

 Devonian. 



43. AscocERAS, Barrande, 1846. = Cryptoceras, Barrande, 1846. 



Shell slightly curved, flask-shaped, lower portion more globular than the upper ; 

 partitions of the chambers few and running parallel to the long axis of the shell, leaving 

 a space the whole length of the shell for the body of the creature ; aperture of exterior 

 chamber not lobed ; Silurian. 



44. Glossoceras, Barrande, 1867. 



Shell same as in Ascoceras, but with the aperture of the exterior chamber lobed ; 

 Silurian. 



45. Aphragmites, Barrande, 1867. 



Shell same in general form as Ascoceras, but without several internal partitions ; 

 aperture not lobed ; Silurian. 



