66 



GEOLOGY. 



kind of Squid, or Cuttlefish, which, when perfect, had a chambered 

 upper portion to its shell (called the " phragmocone"), and a pearly 

 extension beyond (called the pro-ostracum). Some nearly perfect 

 examples have been found in the Lias and Oxford Clay (see Table- 

 cases). The arms were provided with hooklets as well as suckers for 

 holding fast its prey, and each animal had an ink-bag that secreted 

 an inky fluid (known as Sepia, and used as a pigment by artists), 

 which could be ejected into the water at pleasure, so as to conceal the 

 animal's retreat by a cloud of inky blackness. 



They all had strong horny or shelly mandibles, resembling a 

 parrot's beak ; these are frequently met with in a fossil state. 



By far the largest proportion of the fossil forms, however, belong 

 to the Tetrabranchiate, or four-gilled division, represented at the 

 present day by the " Pearly Nautilus " of the Indian Ocean. These- 

 were less active forms than the Squids and Cuttlefishes ; and instead 

 of having, like them, an internal shell, they had a strong external one 

 with a pearly lining, in the large body-chamber of which the soft 

 parts of the animal were enclosed. The rest of the shell is divided 

 by septa, or partitions, into a series of chambers usually filled with 

 fluid, through which a tube passes called the " siphuncle." 



All the beautiful and varied forms of Turrilites, Baculites, Ammon- 

 ites, Ceratites, Goniaiites, Orthoceratites, &c, belong to this great 

 division of the Cephalopoda. 



The shells of the Pearly Nautilus have been obtained in large 

 numbers from the London Clay of Highgate, Hampstead, and the 

 Isle of Sheppey'; the Ammonites in infinite variety of pattern occur 

 from the close of the Cretaceous period to the base of the Secondary 

 rocks ; the Ceratites in the Trias, and the Goniatites in the Carbon- 

 iferous formations, being only modifications of the shells of the same 

 family. 



The older forms chiefly belong to the straight Orthoceratites* 

 having shells like a Nautilus uncurled and straightened out, or to- 

 curious forms, having various degrees of curvature in the shell, 

 between the straight Orthoceras and the involute Nautilus and 

 Ammonite. These variations are also found in many genera of 

 Cephalopod Shells of the Chalk period. A fuller description of the 

 contents of this Gallery will be given in a new Edition of the Guide 

 as soon as the Wall-cases are arranged, but they are not yet available 

 for the reception of specimens. 



Class 2. Pteropoda (wing-shells). — A. single Table-case is devoted 

 to this curious division of Mollusca, represented at the present day 

 by small oceanic animals, whose entire life is passed in the open sea 

 far away from any land, swimming by means of two wing-like 

 appendages, one on each side of the head). The Pteropods had 

 their representatives far back in past geological time. 



In the Miocene beds of Bordeaux, Dax, Turin, Sicily, and in the 

 Suffolk Crag, small delicate shells occur, like existing genera, such as 

 Hyalea, VagineJla, Cuvieria ; whilst in the Carboniferous, Devonian, 

 and Silurian many species are met with, as Conularia, Hyolithes, 



