OWEN ON THE BELEMNITE. 121 



unable to resist ; its basal part is usually squeezed flat, but some- 

 times one of the septa has slipped forwards so as to present its 

 surfaces to the plane of pressure, and so retain its original form. 



The septa are composed chiefly of nacre, with a thinner layer of 

 white friable calcareous matter on both surfaces, which is seldom 

 preserved. I have found twenty septa in an extent of two inches 

 from the base of the phragmocone ; about an equal number of 

 septa dividing chambers progressively diminishing in depth, and 

 more rapidly in width, are indicated by detached phragmocones to 

 have extended to the apex of the socket of the guard. The cap- 

 sule of the phragmocone consists of a thin layer of mixed albu- 

 minous and opaque Calcareous matter lined with nacre, but with a 

 yellowish smooth outer surface. 



The entire phragmocone, with its capsule, of these Belemnites 

 from the Oxford clay, has been found not unfrequently isolated and 

 detached, having slipt out of the alveolar cavity of the guard, and 

 such specimens are squeezed flatter than those which have re- 

 mained in, and been protected by, the guard. The yielding texture 

 of the phragmoconic capsule has commonly caused it to fall into 

 longitudinal folds when compressed after having become detached 

 from the alveolus ; and if the change of form caused by compression 

 were not borne in mind, the piles of concave plates would seem not 

 to have been adapted to the alveolus. 



It was long ago suggested (by Piatt, in 1764,) that the shell of 

 the Belemnite was due to the formative forces of the mantle of a 

 molluscous animal, and the subsequent discovery of two grades of 

 organization in the class of Cephalopoda* (the only mollusca to which 

 it seemed possible to refer this fossil) called for a closer investi- 

 gation, and led to the definite approximation of these with the 

 other families of siphoniferous Cephalopods, now ranked under two 

 distinct orders. 



The first evidence that bore directly on the question of the po- 

 sition of the Belemnite in this class was the discovery of the fossil 

 ink-bag preserved in the basal chamber of the phragmocone of 

 one of these animals. 



The importance of this discovery depended chiefly on the facts 

 that the secreting gland and reservoir of the inky secretion common 

 to all the naked Cephalopods do not exist in the recent Nautilus 

 pompilius, and that no trace of them has ever been met with 

 in connection with any of the simple or typical forms of fossil- 

 chambered shells, as Orthoceratites, Baculites, Ammonites, &c. 

 It appeared, on consideration, that the Nautilus might derive suf- 

 ficient protection from its strong external shell, while, on the other 

 hand, the active and highly organized naked Cephalopods might 

 well require a compensatory endowment, enabling them to escape 

 from danger. The presence of an ink-bladder therefore, since the 

 branchial character of the naked Cephalapods is an essential con- 



* Owen, " Memoir on the Pearly Nautilus." 



