30 THE SHELL. 



sucker at their junctiou. A small portion of the intestinal sack 

 occupies the last chamber of the shell, and a prolongation of it 

 connects the chambers by passing through the siphonal tubes 

 which penetrate the septa towards their inner margin (instead 

 of in the middle, as in Nautilus). 



In the fossil Belemnites, the siphunculated, chambered portion 

 of the shell has been called the phragmoconus, by Owen; the 

 horny or chalky blade is termed, by Huxley, the pro-ostracum, 

 and the rostrum of the latter author corresponds with the 

 similar term heretofore used by us (ii, 19-21). 



Analysis shows the horny shell to be principally composed of 

 chitin. The Sepia officinalis, according to J. F. John, yields of 

 Carbonate of Lime, with a trace of Phosphate, 85'^ ; Water, 4° ; 

 Organic matter, 4° ; Residuum, Magnesia, etc., '7°. 



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

 Ammonites are not tetrabranchiate Cephalopoda, allied to the 

 Nautili, but dibranchiate decapods, having the greatest affinity 

 to the Spirulffi. As early as 1867, Barrande had shown the small 

 resemblance that exists between the Goniatites and the Nautilidse, 

 during the first period of their development. The initial chamber 

 of the phragmostracum in the Nautilidte, does not sensibly differ, 

 in its general organization, from the other primary chambers 

 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 phragmos- 

 tracum of all the dibranchiate 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 Po7npilius, Deroceras planicosta, and the Goniatites, 

 come in support of these observations. Mr. Hj&tt, 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 

 Nautilus Pompilius. The comparative examination which M. 

 Munier-Chalmas has made of the ovisacs of Spirula 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 csecal 

 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- 



