14 KXTERNAL SIIKU,. 



three, or four sm.'ill subdivisions nl its point of insertion upon 

 tin- inner wall. 



The preseiiee (.fan ovisac has been ascertained by M. Chahnas 

 in a number of fossil cephalopods. IJelemnites. A ininonites. ( 'era- 

 titefi, etc. H is iivnerally s|)heroi(lal when the turns of the spire 

 are tree, and ovoid when they are contiguous. But in the living 

 ten-abranchiate cephalopoda, as well as in the remains of tin? 

 many extinct speeies. the presence of an ovisae has never been 

 detected. In Xinililti* and Aturia. the siphon originates upon 

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

 its posterior extremity, by a part of the calcareous prolongation 

 of the septum, which assists in its formation. The external 

 t ransverse cicat rix observed by Mi 1 . Hyatt, can never have been 

 in communication with the siphon ; its purpose is still completely 

 unknown. It has been indicated, by M . l>arrande. upon a great 

 number of Silurian tet rabranchiata. 



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

 epoch the tetrabranchiat.- cephalopoda were as clearly separated 

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

 fications that we can recognize are of generic rank; in fact, the 

 Ammonites, which, when young, have septa like those of I>ero- 

 ceras and ( Joiiiat ites, appear to be derived from one of those 

 \\\w.**Ann. .]/"</ A'. ///>/.. 4th ser.. xiii, l-4, 1874 (from 

 Gomptes Refc<Jtt, is::',). 



K.rlcriut'l Mi I'll. 



Ili-arding the testaceous nest, of the female Argonaut as a 

 -hell, it is the only genus which is unilocula r : in all the others 

 the external shells being divided by partitions into chambers, 

 connected by a siphon. The A rgoiiaut a, of a peculiar fibrous, 

 cornet. -calcareous texture, is distinguished by the want of a 

 nucleus in its infancy, and by its composition of two layers, one 



. lir^i in hi> " Synopsis of the liritish Museum,'' 1S40, and after- 

 ward- in Ann. .I/,/./. A'. Hist., \v, 1S4.1. lias exj.i-esscd the opinion that the 

 f.i.sjsil . \iaiiK.iii1rs \\rrc internal shells, like S]>irula, and consequently, 

 dibranchiatea instead of tetrabranchiates ; and the dill'ei-ent, phm of the 

 inUial rliainlii-r, as justly 'l>ser\ c<l hy MunieM 'halnias, I'.arrande, Hyatt 

 and Kisrlirr. is ron-oli. rat iye !' t his. I d< not yent lire io change t lie jtosi- 

 t ion oi tl ,> losMls, and do not t hink any change desirable until wo shall 

 be able l<> understand their history nioi-e completely. 



