TNTKRNAT- SHELL. 13 



M. Munier-Chalraas has recently endeavored to prove that th(; 

 Ammonites are not tetrahranchiate cephalopoda, allied to the 

 Nautili, but dibranchiate decapods, having the greatest aflinit3' 

 to the Spivul}<?. As earlj-as 18r)7,Barrande had shown the small 

 resemblance that exists between the Goniatites and the Nautilidie, 

 during the first pei'iod of their development. The initial cham- 

 ber of the phragmostraciim in the Nautilida', does not sensibly' 

 differ, in its general oi'ganization, 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 er/c/, 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 dibranchiate mollusca that M. Munier-Chalmas 

 has been able to study. Mr. Alpheus H3'att's very interesting 

 investigations upon the embryogeny of the phragmostracum of 

 Nautilus Po'mjnlius, Deroceras plmiicosto,, 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 sui)position, he adduces the transverse 

 external cicatrix whicii he observed on the initial chamber of 

 Nautilus Pompilius. The comparative examination which M. 

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

 of Ammonites 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 Cfecal 

 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, ojjposite 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 T)y a membrane, which 

 is sometimes simply spread out as in Spirvla Peronii^ or which 

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



