CEPHALOPODA. 



315 



to the museum : here it was anatomised, and the description with 

 copious and beautful illustrations published by the Council.* 



The dissection of this unique specimen established the claims of the 

 Nautilus Pompilius to rank in the highest class of Mollusca, and at 

 the same time brought to light so many important modifications in 

 the cephalopodic type of structure as to necessitate the establishment 

 of a new order for its reception. I propose to devote the present 

 Lecture to the demonstration of the principal organic characters of 

 this order, which I have called Tetrabra?ichiata, and to a brief review 

 of the extinct chambered siphoniferous shells, and of their relations 

 to the existing Cephalopoda. 



The soft parts of the pearly Nautilus (Jiff. 129.) form an oblong 



Nautilus Pompilius. 



mass divided by an irregular transverse constriction into two nearly 

 equal segments ; the posterior is smoothly rounded, soft, and membra- 

 nous, containing the viscera, and adapted to the last chamber of the 

 shell ; the anterior is densely muscular, and includes the organs of 

 sense and locomotion. 



The mantle is very thin upon the posterior part of the body ; it is 

 continued backwards in the form of a slender tube, which penetrates 

 the calcareous siphon (c), in the septum closing the occupied chamber 

 behind, and is thence continued, as the membranous siphon (c?), 

 through all the other divisions of the shell to the central nucleus. As 

 the mantle advances tow^ards the anterior part of the abdomen, it in- 

 creases in thickness, becomes more muscular, and extends freely out- 



* Memoir on the Pearly Nautilus, 4to. 1832. 



