CEPHALOPODA. 



91 



CYRTOCERAS, Goldf. 1533. 

 Etym., cv.rtos, curved, ceras, horn. 



Syn., Campulites, Desh. 1832 (including gyroceras). Aploceras, I>'Orb. 

 Cainpyloceras and trigonoceras, McCoy. 



Ex., C. hybridum, volborthi and beaumonti (Barrande). 

 Shell, curved ; siphuncle small, internal, or sub-central. 

 Distr., 36 sp. L. Silurian, Carb X. America, and Europe. 



Fig. 52.* 



GTROCERAS, Meyer, 1829. 

 Etym., gyros, a circle, and ceras. 

 Sy/i., Nautiloceras, D'Orb. 



Ex., G. eifeliense, D'Arch., pi. II., fig. 13. Devonian, Eifel. 

 Shell, natitiloid ; whirls separate ; siphnncle excentrie, radiated. 

 Fossil, 17 sp. U. Silurian Trias? X. America, and Europe, 



ASCOCERAS, Barrande, 1848.f 



Etym., ascos, a leather bottle, 



Shell, bent upon itself, like ptychoceras. 



Di&tr. t 7 sp. U. Silurian, Bohemia. 



FAMILY III. AinioxiTiDJC. 



Shell. Body-chamber elongated; aperture guarded by processes, and 

 closed by an operculum ; sutures angulated, or lobed and foliated ; siphuncle 

 external (dorsal, as regards the shell). 



The shell of the ammonitida has essentially the same structure with the 

 nautilus. It consists of an external porcellanous^ layer, formed by the collar 



* Fig. 52. Gyroceras goldfussii (= ornatum Goldf). 6. Siphuncle of G. depress**, 

 Goldf. sp. Devonian. Eifel. From M.M. D'Archiac and Verneuil. 



t In Haidinger's Berichte. * 



J Its microscopic structure has not been satisfactorily examined : Prof. Forbes 

 detected a punctate structure in one species. 



