EXTERNAL -HIM. 



neaiv-t llie'^urface beini: tin- largest. The internal or i 

 l:i\ er i- aKo di->l in'' I.V cellular. 



The body <>!' the :iniin:il ..!' Nautilus, which is applied lo Mini 

 forms tin- septum. U of similar -im>oihand rounded form, and 

 \\e may rea-on from analogy t hat t he a rboreseeiit lobes if the 

 septum in tlir fo>sil Ammonites, were likewise moulded up<>n :i 

 similarly formed body. I f we seek :i reason for this complication 



of structure in Ammonites. \vc slirdl find :i possible explanation 



ill tlu> romuTlii)<r siphon wliirh, picicin^ Ihc ccntr^ of tin- si-pi.-i 

 in Nautilus. cn:il>lrs the animal lhrndi\ to st rcii^t h.-n iN hold 

 upon its shell; l>ul piercing laterally in A mmoiiito. '/\\ -es only 

 an e\et ntrie attachment. In the latter case, the loln-> and rami- 

 fications, applied to the corrcs|tondin^ parts of t he animal, would 

 tighten its attachment, and partially remedy the want of the 

 central support. 



The outlines of the septa aivtermrd s//////vs, and when II 

 are fold'<l. as in Ammonites, etc.. the elevations are called 

 xf//*//fs. and the dei>ressi >ns. /oAfx. 'This frilling or foldinu' is 

 found principally at the junction of the septum with the shell- 

 Avail, it l>ein^ nearly Hat in the middle, even when its siitural 

 complication is extreme. These parts are subdivided thus: A 

 MiiLi'le d<>r.il luhf surrounds the siphon, and occiip'u-s t.he nu'dial 

 region <f t he liack. and the additional two lobes on either side of 

 it are named the .>-///>o-/o/- lu/i-rnl /o/'x and i u I'rrinr liilt-i-nl /./Ars. 

 whilst other lolies. \\ hat,ev<-i l their number, are called auxiliary 

 lobes: finally, Opposed to the tlm-tnl is the rrnfriil /<>/'. The 

 saldles bear a corresponding nomenclature, that between the 

 d<.i-->al lobe and the superior lateral, beino- termed the iltn'*til 

 MiiMli-. the next the In/mil >///////. etc. 



All miillilociilar cephalopo.U ha\'e the chambers of their ^hell> 

 eoiinected b\ a ->iphoi, which traverses the <ept:i. :iud receives 

 from tlie latter a series of short iiivesi in^ tubes. 



'flic siphon pierces the middle of the septa in Nautilus, in 

 < )rtlioeer:it ile-, etc.. whilst it is near the internal edue in Auanide-. 

 and the dorsal ed-v in ( Joiiiat ites. Ammonites, etc. 



It ha- been Mipposed by I IiieUluiid. that the siphon, which 



e-'mmiinicates with a large Back, discovered by < )wen, in the bo,|\ 



ot' \tnililiifi r<>,,i/>il/>f.<. may constitute a hydraulic apparatu- l>\ 

 whirli the animal ma\ va r\ t he weight of its shell at will b\ 



