CEPHALOPODA. 



79 



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

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

 variable ; in the ammon'didcB it is external, or close to the outer margin of 

 the shell (fig. 37). luthe nautiUdce it is usually central (fig. 35), or internal 

 (%. 36j. 



Fig. 35. Nautilus. Fig. 36. Clymenia. Fig. 37. ri'iint'-^.^' 



The air-chambers of the recent nautilus are lined by a very thin, li\aDg 

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

 lar lining, connected vd\h. the animal by spaces between the beads of the 

 siphuncle. t 



The bodij-chamher 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. 

 89), 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. Goniatiies.X 



♦ Fig. 35. Nautilus pompilius, L. Fig. 36. Clymenia striata, Miiust., see pi. II., 

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



f The apocryphal genus spougarium, was founded on detached septa of an orih^i- 

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

 from tiie siphuncle. Mr. Jones, warden of Clun Hospital, has several of these in 

 apposition. 



J Fig. 38. Section of ammonites ohtusus, Sby. lias, Lyme Regis; from a verj' young 

 specimen. Fig. 39. Section of goniatites sphcericus, Sby. carb. limestone, Bolland (in 

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

 chamber. 



