14 EXTERNAL SHKL1-. 



three, or four snmll snbdiA'isions :it its point of insertion upon 

 the ijiner wall. 



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

 in a number of fossil cephalopods, Belenmites, Ammonites, Cera- 

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

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

 tetrabranchiate eei)halopoda, as well as in the remains of tlie 

 man}- extinct species, the presence of an ovisac has never been 

 detected. In Nautilus and Aliiria^ the siphon oi-iginates upon 

 the inner walls of the first chamber. It is completelj'- 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. Bnrrandc. upon a greal 

 number of Silurian tetrabranchiata. 



Thus it results, from these ol)servations, that at the Silurian 

 epoch the tetral)ranchiate cephalopoda were as clearly sejia rated 

 from the dibranchiates, as at the i)resent day. The only modi- 

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

 Ammonites, which, when young, have septa like those of Dero- 

 ceras and Goniatites, appear to be diM-ived from one of those 

 types.*— ^)i». Maij. N. Hist., 4th ser.. xiii. IS I, 1ST4 (from 

 Gomples /{,'V(lu.<, ist8). 



External SJh'JI. 



Regarding llie testaceous nest of the female Argonaut as a 

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

 the external shells being divided by partitions into chambers, 

 connected b}- a siphon. The Argonauta, of a i)eculiar fibrous, 

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

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



* Gray, first in his " Synopsis of the British Museum," 1840, and after- 

 wards in Ann. Marj. N. Hist., xv, 1845, has expressed the opinion that the 

 fossil Aminonites were internal shells, like Spirula, and consequently, 

 dibranchiates instead of tetrabrancliiates ; and the difterent plan of the 

 initial chamber, as justly observed by Munier-Chalmas, Barrande, 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 altle to understand tlicir Jiistory more comi>letcly. 



