CONTRIVANCES IN NAUTILUS SYPHO. 271 



As the place of the siphon in this species is upon the inter- 

 nal margin of the transverse plates (PI. 43, Fig. 2, b', b^, b^) 

 it had less power than the more central siphuncle of the 

 Nautilus- to attach the mantle of the animal to the bottom of 

 the outer chamber. For this defect we find a compensation? 

 resembling that which Von Buch considers to have been 

 afforded by the lobes of Ammonites to the inhabitants of 

 those shells. This compensation will be illustrated by a 

 comparison of the lobes in N. Sypho (PI. 43, Fig. 2.,) with 

 a similar provision in the Nautilus Zic Zac (PI. 43, Figs. 

 ^, 4.*) 



space within it for tiie distension of the siphuncle, by a sufRcient quantity 

 of fluid to cause the aniuial to sink. 



At each articulation of the funnels, the diameter of the siphuncle is con- 

 tracted, as the siphuncles of Ammonites and Nautili are contracted at their 

 passage througli the collars of their transverse plates. 



Another point in the organization of the siphuncle is illustrated by this 

 shell, namely, the existence of a soft calcareous sheath, (PI. 43, ¥'ig. 1 , b, c. 

 d.,) analogous to that of the N. Pompilius, (PI. 31, Fig. 1, a. b. c. d.,) between 

 each shelly funnel and the membranous pipe or siphuncle enclosed within it. 

 At PI. 43, Fig. 1, b, we have a section of this sheath folding round the 

 Sinaller extremity of the funnel a'. From c. to d, it lines the inside of the 

 subjacent funnel a^; and from d. continues downwards to the termination of 

 the funnel a2, on the inside of e. At c, and f, we see the upper termination 

 of two perfect sheaths, similar to that of which a section is represented at b. 

 c. d. This sheath, from its insertion between the point of the upper siphon 

 and mouth of the lower one, (Fig. 1, c.,) must have acted as a collar, inter- 

 cepting all communication between the interior of the shelly siphuncular 

 tube and the air-chambers. The area of this shelly tube probably may be 

 sufficient, not only to have contained the distended siphuncle, but also to 

 ;^llo,w it to be surrounded with a volume of air, the elasticity of which would 

 act in forcing back the pericardial fluid from the siphuncle, in the same 

 manner as we have supposed the air to act within the chambers of the N . 

 Pompilius. 



* On each side of the transverse plate in both these species there is 

 an undulation, or sinus, producing lobes (I'l. 43, Fig. 2. a', a^, a^, Fig. 

 3. a. and Fig. 4. a. b.) There is also a deep backward curvature of the 

 two ventral lobes, Fig. 4. c. c. All these lobes may have acted con- 

 jointly witli the siphuncle, to give firm attachment to the mantle of the 

 animal at the bottom of the outer chamber. The shell Fig, 1. is brokeu. 



