THE SHELL. 31 



cation therewith. It is very variable in its general form, and 

 may present strongly marked examples of dimorphism in the 

 same species of Ammonite, It is formed by a membrane, which 

 is sometimes simply spread out as in Spirula Peronii, or which 

 may form a more or less circular tube. It also presents two, 

 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 cephalopods, 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 upoii 

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

 its postei'ior extremitj^, 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 Mr. Barrande, upon a great 

 number of Silurian tetrabranchiata. 



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

 epoch the tetrabranchiate 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 Dero- 

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

 types. — Ann. Mag. N. Hist., 4th ser., xiii, 184, 18T4 (from 

 Comptes Bendus, 18*73). 



External Shell in the Cephalopoda. Regarding the testaceous 

 nest of the female Argonaut as a shell, it is the only cephalopod 

 genus which is unilocular; in all others the cavity of the external 

 shells is divided b}^ partitions into chambers, connected by a 

 siphon. The Argonaut shell (xxiv, 19), of a peculiar fibrous, 

 corueo-calcareous texture,' is distinguished by the want of a 

 distinct nucleus in its infancy, and by its composition of two 

 layers, one placed upon the other. It is secreted by the palmate 

 arms of the female, which are constantly applied to its sides 

 and envelop them (xxiv, 20 ). In the male Argonaut, always 

 much smaller, there are no palmated arms (xvi, 84, 85), and 

 consequently, no shell. The shell itself appears to be useful 

 only as a portable cradle for the development of the eggs (xviii, 

 15), and the animal which forms it does not appear to differ 

 greatly from the shell-less Octopus. 



The question of the parasitism of the animal of the Argonaut 

 in its shell, originally assumed by distinguished naturalists, has 

 been so long debated, that quite a literature upon the subject has 



