MOLLUSCA — CEPHALOPODA. 
161 
as in rotcrioccras (Fig. 91 A). But on one side the septa 
bent upwards, sliutting oft’ curved chambers at the side of 
the shell (Fig. 91 B, C, D). Sometimes the hinder part of 
the visceral cone shrank a little more and again formed some 
ordinary septa above these curiously curved ones (Fig. 91 E). 
Simpler forms of straight or slightly curved shell, with 
transverse septa, the siphunele near the centre, and a plain 
opening in the body-chamber, are grouped under the name 
Orthoccms (Fig. 80). Shells of this nature are found in all 
rocks from Cambrian to Trias, but especially in those of 
Silurian age. Some of them were several feet long. Among 
the many species exhibited one may note the common 
0. luildific from the Lower Ludlow beds; several from 
Bohemia, among which is 0. truncatum, a form that seems to 
have made a practice of dropping the earlier chambers and 
sealing up the broken end of the shell with a plug of shelly 
substance ; some polished sections from the INIiddle Devonian 
limestones of South Devon ; and elongate shells, such as 
0. gracile, from the Lower Devonian of Nassau. Of Devonian 
age are probably the large specimens of 0. chincnse, known to 
tile Chinese as “Pagoda stones,” from the belief that they 
are formed underground wliere the shadow of a pagoda 
has fallen upon the surface. Polished slabs of rock con- 
taining these and other species of Ortlioceras are on the wall 
by the door. 
Slightly curved forms of simple Nautiloid type were 
formerly grouped under the name Cyrtoccras, but these are 
now distributed among several genera of early Palaeozoic age, 
tlie name Cyrtoceras being restricted to a purely Devonian 
genus. At an early period appear more closely coiled shells. 
Several in which the coils or whorls of the shell were 
scarcely, if at all, in contact, were formerly grouped as 
Gyroceras ; but these also are now placed in several distinct 
genera. In some the earlier coils are close, but tlie last 
formed part of the shell is less close, as in the Silurian 
Ophidioccms (Fig. 92 a), or even straight, as though unwound, 
as in the Ordovician Litwitcs. In these two genera the shell 
aperture is contracted. In Trochoceras also the shell is not 
closely coiled, its special featui'e, however, is that the coils 
are not in one plane, but rise in a spire, something like a 
snail-.shell. Begiiming in the Cambrian, this genus lasts to 
I )evonian times, but is most abundant in the Silurian rocks 
gf Bohemia, England, and the United States. Trocliolitcs, an 
Ordovician genus from North America, Europe, and India, 
M 
Gallery 
VII. 
Wall-case 
14 . 
Wall-case 
1 . 
Table-case 
2 . 
