There are also many univalve and bivalve shells of existing genera in the Mountain limestone, such as Turritella, Buccinum, Patella, Isocardia, Nucula, and Pecten.[341-A] But the Cephalopoda depart, in general, more widely from living forms, some being generically distinct from all those found in strata newer than the coal. In this number may be mentioned Orthoceras, a siphuncled and chambered shell, like a Nautilus uncoiled and straightened. Some species of this genus are several feet long ([fig. 392.]). The Goniatite is another genus, nearly allied to the Ammonite, from which it differs in having the lobes of the septa free from lateral denticulations, or crenatures; so that the outline of these is continuous and uninterrupted (see a, [fig. 393.]). Their siphon is small, and in the form of the striæ of growth they resemble Nautili. Another extinct generic form of Cephalopod, abounding in the Mountain limestone, and not found in strata of later date, is the Bellerophon ([fig. 394.]), of which the shell, like the living Argonaut, was without chambers.
Fig. 393.
Goniatites evolutus, Phillips.[342-A] Mountain limestone.
Fig. 394.
Bellerophon costatus, Sow.[342-B] Mountain limestone.