CEPHALOPODA. 



79 



its centre a smaller tube, tlie space between the two being filled up with radiat- 

 ing plates, like the lamellaj of a coral. The position of the siphuncle is very 

 variable ; in the ammonitida it is external, or close to the outer margin of 

 the shell (fig. 37). In the nautilidae it is usually central (fig. 35), or internal 

 (% 36). 



Fig. 35. Nautilus. Fig. 36. ' Clymenia. Fig. 37. Hamites* 



The air-chambers of the recent nautilus are lined by a very thin, living 

 membrane ; those of the fossil orthocerata retain indications of a thick vascu- 

 lar lining, connected with the animal by spaces between the beads of the 

 siphuncle. f 



The body-chamber is always very capacious ; in the recent nautilus its 

 cavity is twice as large as the whole series of air-cells ; in the goniatite (fig. 

 39), it occupies a whole whirl, and has a considerable lateral extension ; and 

 in ammonites communis it occupies more than a whirl. 



Fig. 38. Ammonites. Fig. 39. Goniatites.% 



* Fig. 35. Nautilus pompilius, L. Fig. 36. Clymenia striata, Miinst., see pi. II., 

 fig. 16. Fig. 37. Hamites cylindraceus Defr., see fig. 58. 



t The apocryphal genus spongarium, was founded on detached septa of an ortho- 

 ceras, from the Upper Ludlow rock, in which the vascular markings distinctly radiate 

 from the siphuncle. Mr. Jones, warden of Chin Hospital, has several of these in 

 apposition. 



I Fig. 38. Section of ammonites obtusus, Sby. lias, Lyme Regis ; from a very young 

 specimen. Fig. 39. Section of goniatites sphtericus, Shy. carb. limestone, Bolland (in 

 the cabinet of Mr. Tennant.) The dotted lines indicate the lateral extent of the body- 

 chamber. 



