INTERNAL SHELL. 13 



M. Munier-Chalmas lias recently endeavored to prove that the 

 Ammonites are riot tetrabranchiate cephalopoda, allied to the 

 Nautili, but dibrancliiate decapods, having the greatest affinity 

 to the Spiruhi 1 . As early as 186T,Barrande had shown the small 

 resemblance that exists between the Goniatites and the Nautilidas 

 during the first period of their development. The initial cham- 

 ber of the phragmostracum in the Nautilidae, does not sensibly 

 ditto r, in its general organization, from the other primary cham- 

 bers which are developed a little later; whereas the initial shell 

 of the Goniatites appears in the form of an egg, isolated from the 

 first air-chamber by a distinct constriction. This initial chamber 

 or ovisac, of the Goniatites, so different from those which imme- 

 diately succeed it, is met with at the origin of the phragmostra- 

 cum of all the dibrancliiate mollusca that M. Munier-Chalmas 

 has been able to study. Mr. Alpheus Hyatt's very interesting- 

 investigations upon the embryogeny of the phragmostracum of 

 Nautilus Pompilitui) Deroceras planicosta, and the Goniatites, 

 come in support of these observations. Mr. Hyatt, however, 

 preoccupied by his theoretical ideas upon the evolution of living 

 creatures, in order to establish the affiliation of the Ammonites 

 and Nautili, supposes that the latter lost their ovisac by trunca- 

 tion. To support this supposition, he adduces the transverse 

 external cicatrix which he observed on the initial chamber of 

 \aniilns Pompilius. The comparative examination which M. 

 Munier-Chalmas has made of the ovisacs of Spirilla Peronii and 

 of Ammonite* Parkinsoni, and other species, has shown that in 

 these mollusks the siphon originates in the ovisac a little before 

 the appearance of the first septum. It commences by a caeca! 

 inflation, which bears the prosiphon in its prolongation. The 

 new organ, to which he gives the name of prosiphon, must take 

 the place of the siphon during the embryonic period. It 

 originates in the ovisac, opposite the siphonal inflation, upon 

 which it terminates, but without having any internal communi- 

 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, 



