CEPHALOPODA. 189. 



has taken advantage of these characters for dividing the nautili 

 into three groups, viz., 1. LcevigaU. Nautili with smooth shells 

 ranging from the Permian epoch to the present time ; 2. Radiati. 

 Shells ornamented with transverse ribs, mainly cretaceous ; 

 and 3. Striati. Shells ornamented with longitudinal striae. 

 These are confined to the oolite in Europe. In India a few 

 species occur in the lower chalk. 



Suh-genus. Aturia (Bronn). = Megasiphonia, D'Orb. 



Type, N. zic-zac, Sby. PI. 11., Kg. 12, London Clay, 

 Highgate. 



Shell, sutures with a deep lateral lobe ; siphuncle nearly 

 internal, large, continuous, resembling a succession of fannels. 



Fossil, 4 species. Eocone ; North America, Europe, India. 



Suh-genus ? Discites, McCoy. Wliorls all exposed ; the last 

 chamber sometimes produced. 



Fossil, 6 species. Lower Silurian. — Carb. limestone. 



Temnocheilus, McCoy. Founded on the carinated species of 

 the carb. limestone, of which 5 are known. 



Cryptoceras, D'Orb. Ascoceras, Barr. Pounded on K. dor- 

 salis, Phil., and one other species, in which the siphuncle is 

 nearly external. * 



Fossil, 16 species. Upper Silurian — Carb. 



LiTUlTES, Breynius. 



Etymology, lituus, a trumpet. 



Synonyms, Hortolus, Montf. (whorls separate). Trocholites, 

 Conrad. 



Example, L. convolvans, Schl. L. lituus, Hisinger. 



Shell, discoidal ; whorls close or separate ; last chamber pro- 

 duced in a straight line ; siphuncle central or sub -central. 



Fossil, 18 species. Silurian ; North America, Europe. 



Trochoceeas, Barrande, 1848. 



Example, T. trochoides. Bar. 



Shell nautiloid, spiral, depressed. 



Fossil, 44 species. Upper Silurian ; Bohemia. 



Some of the species are nearly flat, and, having the last 

 chamber produced, would formerly have been considered 

 Lituites. 



