184 Miscellaneous. 



fact that this form of the origin of the shell, which occurs also in 

 Orthoceras, appears to be similar in all the types of K^autilidae in 

 which we have observed it hitherto. It contrasts, on the other hand, 

 with the origin of the shell of the Goniatites, which appears in the 

 form of an egg, isolated from the first air-chamber by a distinct con- 

 striction." 



This initial chamber (ovisac) of the Goniatites, so diiferent from 

 those which immediately succeed it, is met with at the origin of the 

 phragmostracum of all the dibranchiate Cephalopoda that I have 

 been able to study. 



The new and very interesting investigations carried on at 

 Philadelphia by Mr. Hyatt, upon the embryogeny of the phragm- 

 ostracum of Nautihis pompilius, Deroceras plankosta, and the 

 Goniatites, come in support of these observations. It must be 

 added, however, that Mr. Hyatt, preoccupied by his theoretical 

 ideas upon the evolution of living creatures, in order to establish the 

 fihation of the Ammonites and Nautili, supposes that the latter lost 

 their ovisac by truncation. To support this supposition, he adduces 

 the transverse external cicatrix which he observed on the initial 

 chamber of Nautilus pompilius. 



The numerous observations which I have since been able to make 

 upon the termination of the siphon in Aturia zigzag, in the Jurassic, 

 Cretaceous, and Tertiary Nautili, and in the three existing species, 

 the microscopic examination of a transverse section of the initial 

 chamber of Nautilus pompilius and N. umhilicatus, and a 

 careful comparison of those tSilurian Cephalopoda which lose the 

 extremity of their phragmostracum by truncation have led me to a 

 result completely opposed to the theoretical views put forward by 

 Mr. Hyatt, but conformable in all points with the facts observed by 

 M. Barrande. 



The comparative examination which I have made of the ovisacs 

 of Spirilla Peroiiii and of Ammonites Parkinsoni, A. ooliticus, A. 

 mamillaris, &c. has shown me the relations which exist between 

 these two types during their embryonic evolution. In fact, in the 

 Spirulce and the Ammonites 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 I give the name oi 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 communication therewith. It is very variable 

 in its general form, and may present a strongly marked example 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 

 sometimes presents two, three, or four small subdivisions at its point 

 of insertion upon the inner wall. 



I have ascertained the presence of an ovisac in the genera 

 Belemnites, Belem,nitel.la, Beloptera, Belopterina, Spirulirostra, Am- 

 monites, and Ceratites. In Deroceras, Clymenia, and Goniatites its 



