CEPHALOPODA. 97 



as carnivorous Mollusks, in the Tertiary and recent periods 

 by the pectinibranchiate Gastropods. 



The organization of the pearly Nautilus (Nautilus Pom- 

 pilius) throws light upon that of the extinct Ammonites, Ortho- 

 ceratites, Lituites, Turrilites, etc., and possesses, therefore, an 

 extrinsic interest, besides that which arises from the peculiar 

 modifications of molluscous structure which it presents. 



In fig. 29, representing the animal retracted within the 

 shell, a c, shews the chambered part, h the last chamber, a the 

 attaching muscle, c the crop, / the funnel, h the hood, t the 

 tentacles, m the free margin of the retracted mantle. 



Of the lower group of Cephalopods, possessing chambered 

 shells similar to the pearly Nautili, there are 1400 extinct 

 species, belonging to above 30 genera, while 3 or 4 species 

 alone exist in modern seas. These fossils resemble the Nauti- 

 lus, and differ from the dibranchiate Spirula in the structure 

 of their shell, which is composed of two layers, the outer por- 

 cellaneous, the inner pearly ; whereas the Spirula — an internal 

 shell — is entirely nacreous. They also agree with the Nautilus 

 in the relative capacity of their last chamber, which seems 

 obviously large enough to contain the whole animal. More- 

 over, it appears, from the position of the siphuncle and the 

 form of the aperture, that these shells were revolutely spiral, 

 or coiled over the back of the animal, and not involute like 

 the Spirula. No traces of fossil ink (sepia) or horny claws 

 have been found associated with them, nor any indications of 

 dense muscular tissue, even in the same matrix which has pre- 

 served so completely the mummy cuttle-fish. By their form 

 and size they were ill adapted for rapid locomotion, and must 

 have depended for safety on the shelter afforded by their 

 solid shell. The discoidal Ammonites attained a diameter 

 approaching 3 feet, and the straight-shelled Orthoceratites 

 sometimes exceeded 6 feet in length. These latter must 

 have lived habitually in a position nearly vertical; whilst 



H 



