CEPHALOPODA. 



79 



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

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

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

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

 (%-36). 



Fig. 35. Nautilus. Fig. 36. Clymenia, Fig. 37. A /iu?. " 



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

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

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

 sij)huncle.t 



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

 cavity is tmce as large as the whole series of atr-ceUs ; in the goniatite (fig. 

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

 in amnvonites commimis it occupies more than a whiii. 



Fig. 3S. Ammonites. Fig. 39. Goniatites.X 



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

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



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

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

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

 apposition. 



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

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

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

 chamber. 



