578 LECTURE XXIII. 



symmetrical figure, it might, on the one hand, have been objected that 

 the correspondence of outward form alone, without the camerated and 

 siphonated structure, was insufficient to support the conclusion of the 

 cephalopodic nature of the Nautilus or Spirula, since the shell of the 

 Carinaria might have been adduced as much more closely resembling 

 that of the Argonauta ; and, on the other hand, the scepticism of the 

 majority of conchologists at that period, as to the Argonaut shell 

 being actually fabricated by the Cephalopod usually found in it, 

 might have been adduced to show how little value could be attached 

 to the superficial resemblance between the Argonaut shell and that 

 of the Nautilus, in the determination of the Cephalopodic character 

 of the constructor of the latter ; and, by inference, of those of the 

 allied extinct chambered shells. Most acceptable, therefore, at this 

 conjuncture, was the arrival of the molluscous inhabitant of the 

 Nautilus Pompilitis. 



The long-sought-for animal was captured in the South Seas by 

 Mr. George Bennett, a member of the Royal College of Surgeons, 

 and by him presented to the museum of the college : it was entrusted 

 to me to be anatomised, and my description with illustrations was 

 liberally published by the Council.* 



The dissection of this unique specimen established the claims of the 

 Nautilus Pompilius to rank in the highest class of MoUusca, 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. The present Lecture will be devoted 

 to the demonstration of the principal organic characters of this order, 

 which is called Tetrabranchiata^ and to a brief review of the extinct 

 chambered siphoniferous shells, and of their relations to the existing 

 Cephalopods. 



The soft parts of the pearly Nautilus {^g. 214.) form an oblong 

 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 {d) 

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

 the mantle advances towards the anterior part of the abdomen, it in- 



* CCCLXXXVII. 



! 



