V. Fossil Cephalopoda, Nautilus, Ammonite, &c.--The fossil remains of the molluscous animals, named _Cephalopoda_, from their organs of prehension being arranged around the head or upper part of the body, are the most ancient, numerous, and interesting, of this class of animated nature in the mineral kingdom. These relics are among the most varied and striking of the extinct beings that occur in the sedimentary strata, from the most ancient secondary formations, to the most recent tertiary. The living species are but a feeble representation of the countless myriads which must have swarmed in the ancient seas. The animal of the Cephalopods is composed of a body, which is either enclosed in a shell, as in the Nautilus, or contains a calcareous osselet or support, as in the Sepia or Cuttle-fish; it has a distinct head, and eyes as perfect as in the vertebrated animals, with complicated organs of hearing, and a powerful masticating apparatus, surrounded by arms or tentacula. Below the head there is a tube which acts as a locomotive instrument, to propel the animal backwards, by the forcible ejection of the water that has served the purpose of respiration, and which can be ejected with considerable force by the contraction of the body. Their fossil remains consist of the external shell and the internal osselet; and in the naked tribes, of the soft parts of the body, the ink-bag, &c., as noticed in the account of the Belemnite and Belemnoteuthis. The shell varies exceedingly in the different genera. In the group characterised by smooth septa, and a medial or submedial siphuncle, as the Nautilus, the earliest or most ancient type is straight, as in the _Orthoceras_ (Plate LVIII. fig. 14) of the palæozoic formations; the intermediate forms present various modifications of the spiral, and terminate in the completely discoidal shell of the living genus; while the other group, that with sinuous or foliated septa and a dorsal siphuncle, commences in a discoidal type--the Ammonite, which gradually passes through the various modifications of _Crioceras_, _Scaphites_ (Plate LXI. fig. 10), _Hamites_ (Plate LXI. fig. 3), _Turrilites_ (Plate LXI. fig. 12), &c.; and finally becomes extinct in the straight Baculites (Plate LX. fig. 2).