12 GENESIS OF THE AEIETIDiE. 



Nautilus, and in the vast majority of all known fossils of the order so far known 

 to the author this stage had similar characters. The siphon was larger at this 

 stage than subsequently, and possessed a prolongation which reached down into 

 and lined the primitive caicum. This closed pipe was however more or less cylin- 

 drical, and formed a transition to a cylindrical, open, siphonal tube, when com- 

 pared with the caecum on the one hand and the siphon of the succeeding septa 

 on the other. The second septum was prolonged apically into a funnel, and this 

 was continuous with a true porous wall, which formed the remainder of the 

 pouch. We have already pointed out the probability that this wall was the 

 homologue of the calcareous sheaths or endocones which filled the interiors of 

 the siphons of Endoceratida?. There is, therefore, as previously stated by the 

 author, a structural though highly concentrated and much modified remnant of 

 the adult siphonal elements of an Endoceras still preserved in this stage, even in 

 the existing Nautilus, and we propose to name it the Macrosiphonula. 



9. The next na?pionic stage in living Nautilus was the third living chamber 

 and third septum with its siphon. The siphon has a true funnel, and the siphonal 

 wall attached to it is less swollen out, and seems, upon re-examination of the 

 junctions at the opening of the funnel in the second septum, to be discontinuous. 

 If we are correct, this stage has a small siphon consisting of the usual funnel and 

 tubular porous wall, as in the vast majority of all Nautiloids and Ammonoids. We 

 proposed to name this, according to its aspect and structure, the Microsiphonula. 

 The microsiphonula, though a naspionic stage in the modern Nautilus, did not 

 always occur among larval stages, but had in common with the macrosiphonula 

 a traceable beginning in the adult stages of ancestral types. The genesis of the 

 two forms of siphon may be studied in the Endoceratidaa. In this family Cyrto- 

 cerina had a siphon, which continually increased in size, probably throughout life, 

 though more forms need to be described before one can be assured of this as a 

 fact. There is no doubt, however, that the next form, Piloceras, had what we can 

 safely call a macrosiphon of typical structure until very late in life. The large 

 shells collected in Newfoundland by the author had siphons of great size, which 

 were only slightly contracted or remotely approximated to the tubular condition 

 of the Orthoceratida? in the adolescent and adult stages. 



This and other changes occurring in the adolescent stages induced us to 

 distinguish them by a special term, Nealogic. The adolescent or , nealogic 

 stages, therefore, and the stages of the adult, or, as we have named them, the 

 Ephebolic stages, in Piloceras show for the first time a tendency to contract 

 the siphon or approximate to the microsiphonula, but they never had a true 

 microsiphon. The contracted siphon in these forms, as in the other genera of 

 this family, always had the holochoanoidal or complete funnel reaching from one 

 septum to the next, and a series of conical concentric endocones, or sheaths, as 

 they have been called by others, which stretched from the ends of the funnels, 

 and were the homologues of the porous Avails of the segments of the siphon in 

 Nautilus. 



The terminations of the endocones were prolonged into a central tube, or 

 endosiphon, which we have previously described, and which probably served as a 



