CEPHALOPODA. 597 



of the individual. This structure reminds one of Gomphoceras ; to 

 wliich illustration may now be added the more remarkable instance of 

 the Rliizocliilus, a siphonatcd univalve, in which both the outer and 

 iimer lip of the shell-aperture is extended, so as to cement the shell 

 to the axis of a coral {^Antipatlies ericoides), or to another shell, and 

 at length to I'cduce the aperture to the slender tube of the anterior 

 si|>hon of the mantle, — the sole medium of entry and exit of the re- 

 spiratory currents, and of the nutritive and excretory particles. 



The TurriUte is essentially an Ammonite disposed in spiral coils. 

 The Ilamite and Scaphite are other modifications of the outward 

 form of similarly constructed chambered shells : in the former the 

 small extremity of the shell is curved, the rest being straight ; in the 

 latter both ends arc curved towards each other like those of a canoe. 



With none of these species has there ever been found a trace 

 of the ink-bag ; a part, indeed, of so delicate a texture that some 

 surprise may be naturally felt that any evidence of its existence could 

 be expected to be met with in a fossil state. 



I shall, however, conclude this Lecture by bringing before you ex- 

 amples in w^hich not only the ink-bag, but the muscular mantle, the 

 fins, the eyes, and the tentacles and their horny hooks, of extinct 

 Cephalopods have been preserved from the remote periods of the 

 oolitic deposits to the present time. Independently of these and 

 many other examples of the durability of the inky secretion of the 

 gland which is peculiar to the naked Cephalopods, the absence of 

 the organ in the shell-clad Nautilus, too well protected to need such 

 means of temporary concealment, would lead to tlie inference that the 

 ink-bag must have been absent in the other low-organised Cepha- 

 lopods covered by chambered siphoniferous shells. 



A more complicated fossil shell than any of the preceding, but 

 allied to them by the camerated and siphoniferous structure of one 

 of its constituent parts, once occasioned much perplexity amongst 

 palaeontologists ; the evidence of its nature has however for some 

 years been in the main sufficient to determine its affinities, about 

 which there now appears to be no difference of opinion. The shell 

 to which I allude is that called the '" Belcmnite," which is associated 

 with the more obvious congeners of the Nautilus through a consider- 

 able range of the secondary rocks. It makes its first appearance, 

 with Teudopsis and Celano, in the Lias, as the precursors of the 

 Calamaries and Cuttles. 



The chambei'ed part of the shell of this extinct Cephalopod has 

 the form of a straight cone {Ji<J- 218, b), the septa being numerous, 

 with a slight and equable concavity directed towards the outlet or 

 base of the cone. The intervening chambers are so shallow that 



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