NOMENCLATIVE OF STAGES OE GKOWTH AND DECLINE. 15 



point to make in its favor. If the protoconcli of Nautiloids was an empty con- 

 chiolin shell and represented the veliger stage, it most certainly could not have 

 been the ancestral form from which the calcareous tendency of the same stage in 

 Ammonoids was derived. The characteristics of the asiphonula of Nautiloids are, 

 however, just what are needed to fill the gap. The apex at this stage in Nau- 

 tiloids is rounded and calcareous. The tendency to deposit calcareous matter 

 could therefore have been inherited from an ancestor corresponding to the asi- 

 phonula, and which we will name the Asiphonophora. The Asiphonophora must 

 have had a calcareous shell acquired as an adaptive character, without internal 

 calcareous septa or a siphon. This form could not have been by any means so 

 far removed from the ancestor of the veliger as the immediately following an- 

 cestor of the macrosiphonula, which we have named the Macrosiphonophora. 

 This must have had septa and a central axis of caeca, or at any rate at least 

 one septum and a caecum. 



The characters of the Asiphonophora, when transmitted to the Ammonoids 

 according to the law of acceleration, would have been inherited earlier than in 

 Nautiloids, would therefore have affected the growth of the protoconch, and 

 would have necessarily produced the calcined shell of this stage in Ammonoids. 

 The fusion of the protoconch with the conch in all Ammonoids was the imme- 

 diate result of this process, and in this, way the more tubular form and freer 

 connection of the protoconch with the true conch, and the constant adhesion 

 of the former to the latter, can be explained. 



The disappearance of the asiphonula as a distinct stage in the young of 

 the Ammonoids appears to us, therefore, not an argument against the deriva- 

 tion of the Ammonoids from Asiphonophora, but in favor of this opinion. In 

 fact, it seems to us that, in order to disprove it, opponents will have to find 

 a cicatrix upon the apex of the protoconch in the Ammonoidea. According to 

 the uncompromising attitude of those who insist upon the naked facts, and are 

 hostile to explanations, the protoconcli is the apex of the conch in Ammonoids, 

 and the absence of any cicatrix upon the tip of this is a difficulty they can only 

 surmount by asserting that the general and special homologies we have traced, 

 and all the embryological and naspiological correlations, are purely homoplastic, 

 and do not indicate the derivation of the Ammonoids from any form of Nauti- 

 loid. They must also explain away the similarity of the protoconch in external 

 aspect to the veliger shell in Gasteropoda, since this is an earlier stage than 

 that of the apex of the true conch in Nautiloids, Ammonoids, and all cephalous 

 mollusks. Can any of these gentlemen tell us why the cicatrix does not appear 

 upon the protoconch of Ammonoids, and explain at the same time how that 

 shell came to be similar to the veliger shell in the Cephalophorous Mollusca 

 on the one hand, and the apex of the conch of Nautilus on the other? 



It must be observed, also, that we do not insist that the primary radical of 

 the Ammonoids, Anarcestes, was necessarily descended directly from Endoceras, 

 but that it had probably come from a prototype like the veliger, possibly, as 

 suggested by Brooks, from a class now only represented by the genus Dentalium. 

 The next step, according to our translation of the evidences, must have been 



