CEPHALOPODA. 189 
has taken advantage of these characters for dividing the nautili 
into three groups, viz., 1. Levigati. 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 stria. 
These are confined to the oolite in Europe. In India a few 
species occur in the lower chalk. 
Sub-genus. Aturia (Bronn). = Megasiphonia, D’Orb. 
Type, N. zic-zac, Sby. Pl. II., Fig. 12, London Clay, 
Highgate. 
Sheil, sutures with a deep lateral lobe; siphuncle nearly 
internal, large, continuous, resembling a succession of funnels. 
Fossil, 4 species. Hocene; North America, Europe, India. 
Sub-genus 2 Discites, McCoy. Whorls‘all exposed; the last 
chamber sometimes produced. 
Fossil, 5 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. Founded on WN. dor- 
salis, Phil., and one other species, in which the siphuncle is 
nearly external. 
Fossil, 16 species. Upper Silurian—Carb. 
Litvuites, Breynius. 
Etymology, lituus, a trumpet. 
Synonyms, Hortolus, Montf. (whorls separate). Trocholites, 
Conrad. 
Example, L. conyolvans, Schl. IL. 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. 
TROCHOCERAS, 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 haye been considered 
Lituites, 
