14 i.\ i i i; v\ i. 



three, or four small suli<li\ isions at its point of insertion upon 

 1 lie inner wall. 



The presence of an ovisac has been ascertained by M. ( 'lialmas 

 in a number of fossil cephalnpods. IJelenmitcs. A imnonil.-. ( Yra- 

 tites. etc. 11 is generally spheroidal when the turns of the spire 

 are free, and ovoid when they are contiguous, lint in the living 

 tetrabranchiate cephalopoda, as well as in tin- remains of tin- 

 many extinct species, the presence of an ovisac has never been 

 detected. In Naufi/n* : md Aim-in* the siphon originates upon 

 the inner walls of the first, chamber. It is completely closed at. 

 its posterior extremity, by :> part of tin- calcareous prolong! ion 

 (' the septum, which assists in its formation. The external 

 tran&verse cicatrix observed by Mr. Hyatt, ran never have been 

 in communication with the siphon ; its purpose is still completely 

 unknown. It has been indicated, by M. Uai'i'ande. upon a great 

 number of Silurian tet rabrancliiata. 



Thus it results, from these observations, that at the Silurian 

 epoch the let (-abranchiate cephalopoda were as clearlv separated 

 from the dibranchiates. as at the present day. The only modi- 

 fications that we can recogni/c are of generic rank: in fact, the 

 Ammonites, which, when voting, have septa like those of Pero- 

 ceras and (ioniatites. appeal 1 to be derived from one oj' those 

 t\ -pes.* Ann. May. \. 7//x/.. 4th ser., xiii. 1S--I, 1S74 (from 

 1S7M). 



External M<>ll. 



leranliniLi' the testaceous nest of the female Argonaut as a 

 shell, it is the only neims which is nniloeiilar : in all the others 

 the external shells beinu' divided by partitions into chambers, 

 connected bf a si]>hon. The A I'^onanta. of a peenliai' fibrous, 

 corneo-calcareons texture, is dist in^nisheil by the want of a 

 nucleus in its infancy, and by it- composition of two layers, <>nc 



* Gray, lirsl in liis " Svn< >j.sis uf t lie Uritish Museum." 1*40, and after- 

 \\ards in Ann.. Mil;/. *\. OUt. t >-\, I ^ -!.">. lias expressed tlie o])ini<>n that the 

 fossil Ammonites were internal shells, like Sjiinila, and c.n>f.|iiently, 

 diliranrhiates instead ot' tet rabraneliiates ; and the dillerent plan of the 

 initial fhaml>er, as just ly olser\ cd ly Munier-( 'lialmas, liarrande, Uyaii. 

 and Kiselu-r. is conoborat i ve of t his. 1 do not rentUTO to change the pOBi- 

 tion of these fossils, ;ind do not think am chan^t' desirable until \vo shall 

 lie able to understand their history more completely. 



