332 LECTURE XXIII. 



gether by the dove- tailed lobes of the margins of the chambers. The last 

 chamber of the Ammonite was of large size, as in the Nautilus, and 

 doubtless contained all the soft parts of the animal save the small pro- 

 portion which was prolonged into the siphon. Certain sj^ecies, re- 

 cently described by Mr. Pratt from the Oxford clay, were characterised 

 by the production of a long and narrow process from each side of the 

 mouth of the shell, indicating a corresponding modification in the 

 lobes of the mantle, analogous to that which produces the auricular 

 appendages of the mouth of the shell in certain Argonautae. The 

 same gentleman has likewise recently shown me a small Ammonite in 

 which the mouth of the shell is arched over transversely by a convex 

 plate of calcareous matter continued from the lateral margins of the 

 outlet, and dividing this into two apertures, one coiTesponding with 

 that above the hood of the Nautilus, which gives passage to the 

 dorsal fold of the mantle ; the other with that below the hood, whence 

 issue the tentacles, mouth, and funnel : such a modification, we may 

 presume, could not take place before the termination of the growth of 

 the individual. 



The Turrilite is essentially an Ammonite disposed in spiral coils. 

 The Hamites and Scaphites 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 are 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 excited in such of my hearers as are not conversant 

 with the wonders of geology, that any evidence of its existence covdd 

 be expected to be met with in a fossil state. 



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

 amples in which 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 ex- 

 amples of the dui'ability 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 the inference that the ink-bag must have 

 been absent in the other low-organised Cephalopods 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, has occasioned much perplexity amongst 

 Palseontologists ; the evidence of its nature has however for some 



