14 EXTERNAL SHELL. 



three, or four small subdivisions at its point of insertion upon 

 the inner wall. 



The presence of an ovisac has been ascertained by M. Chalmas 

 in a number of fossil cephalopoda, Belemnites, Ammonites, Cera- 

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

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

 tetrabranchiate cephalopoda, as well as in the remains of the 

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

 detected. In Nautilus 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 

 transverse cicatrix observed by Mr. Hyatt, can never have been 

 in communication with the siphon; its purpose is still completely 

 unknown. It has been indicated, by M. Barrandc, upon a great 

 number of Silurian tetrabranchiata. 



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

 epoch the tetrabranchiate cephalopoda were MS 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 Dero- 

 ceras and Goniatites. appear to be derived from one of those 

 types.* Ann. May. JY. ///x/.. 4th ser., xiii. 1*4, 1874 (from 

 Comjtfc* /ti'iidit*, 1873). 



E.rlc.riial Shell. 



Regarding- the testaceous nest of the female Argonaut as a 

 shell, it is the only genus which is unilocular; in all the others 

 the external shells being divided by partitions into chambers, 

 connected by a siphon. The Argonauta, of a peculiar fibrous, 

 corneo-calcareous texture, is distinguished by the want of a 

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



* Gray, iirst in his "Synopsis ol'the British Museum," 1840, and after- 

 wards in Ann. Mag. JV. Hist., xv, lS4r>, lias expressed the opinion that the 

 fossil Ammonites were internal shells, Itke Spiral a, and consequently, 

 dibranehiatcs instead of tctrabranchiates ; and the different plan of the 

 initial chamber, as justly observed by Muni or-C halm as, Han-mule, Hyatt 

 and Fischer, is corroborative of this. I do not venture to change the posi- 

 tion of these fossils, and do not think any change desirable until we shall 

 be able to understand their history more completely. 



