MOLLUSCA — CEPHALOPODA. 



161 



as in Poterioccras (Fig. 91 A). But on one side the septa Gallery 

 bent upwards shutting off 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 U). 



Simpler forms of straight or slightly curved shell, with Wall-case 

 transverse septa, the siphuncle near the centre, and a plain 14. 

 opening in the body-chamber, are grouped under the name 

 Orthoceras (Fig. 80). Shells of this nature are found in all 

 rocks from Cambrian to Trias, but especially in those of 

 Silurian a^e. Some of them were several feet loner. Amonoj 

 the many species exhibited one may note the common 

 0. ludense from the Lower Ludlow beds ; several from 

 Bohemia, among which is 0, truncatnm, 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 Middle Devonian 

 limestones of South Devon ; and elxDngate shells, such as 

 0. gro^cile, from the Lower Devonian of Nassau. Of Devonian 

 age are probably the large specimens of 0. chinense, known to 

 the Chinese as " Pagoda stones," from the belief that they 

 are formed underground where the shadow of a pagoda 

 has fallen upon the surface. Polished slabs of rock con- 

 taining these and other species of Orthoceras are on the wall 

 by the door. 



Slightly curved forms of simple Nautiloid type were Wall-case 

 formerly grouped under the name Cf/rtoccras, but these are 

 now distributed among several genera of early Palaeozoic age, 

 the name Cyrtoceras being restricted to a purely Devonian 

 genus. At an early period appear more closely coiled shells. Table-case 

 Several in which the coils or whorls of the shell were 2. 

 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 the last 

 formed part of the shell is less close, as in the Silurian 

 Ophidioceras (Fig. 92 a), or even straight, as though unwound, 

 as in the Ordovician Lituites. In these two genera the shell 

 aperture is contracted. In Trochoceras also the shell is not 

 closely coiled, its special feature, however, is that the coils 

 are not in one plane, but rise in a spire, something like a 

 snail-shell. Beginning in the Cambrian, this genus lasts to 

 Devonian times, but is most abundant in the Silurian rocks 

 of Bohemia, England, and the United States. Trocholites, an 

 Ordovician genus from North America, Europe, and India, 



M 



