90 JOURNAL OF MARINE ZOOLOGY AND MICROSCOPY. 



true camerated shells by the nacreous layer, must necessarily be 

 absent, though the design of both is the same. Again, the complex 

 foldings of the Ammonite septa show that they were formed from a 

 very loose mantle, and being curved forwards instead of backwards, 

 as in the Nautili, the membrane must have been blown out from 

 behind while it was depositing the shell : also where the siphuncle 

 passes through the septa, the little funnel around it there, is turned 

 forwards, as if sticking into the animal. Hence we must conclude 

 that the mantle, before it secreted the septum, must have been 

 distended with something which at the time of the secretion had 

 disappeared, and the mantle was blown forwards to take up its space, 

 for useless superabundance of mantle is inconceivable. The only 

 suggestion that can be offered, is that this something was eggs/ 16 ) 

 which, accumulating in the hinder portion of the animal, forced it 

 forwards ; when the full complement was laid, and they became 

 extruded, the mantle secreted gas on its outer surface, which blew 

 it out to take the place left empty ; but where the internal organs 

 had once pressed firmly, the mantle adhered to the shell, and a series 

 of festoons were formed which ultimately became translated into the 

 lobes and saddles of the particular species of Ammonite. 



The animal cannot have moved forwards merely to obtain more 

 body space, for in Arcestes, for instance, the rate of increase is so 

 minute, that each living-chamber is inappreciably larger than the one 

 before. Also there would in that case be a drag on the fleshy 

 Siphuncle, which, when the shell was secreted around the proximal 

 part must have formed a backwardly directed funnel ; but on the 

 supposition that the animal was forced forwards by the eggs, the 

 siphuncle would have time to permanently stretch, and the gas 

 would then be able to press it up to its insertion in the body wall 

 and a forwardly directed funnel come to be secreted. 



No such structure as these complicated suture lines could be 

 found in a male, whose spermatozoa do not take up anything like the- 

 space that ova do, which in the Cephalopoda generally contain an 

 enormous amount of yolk. 



Thus we get an additional probability that the Argonaut is 

 closely allied to the Ammonites, since the shell in both is only 

 possessed by the female. Munier-Chalmasl 1 ?) and Haug,( 18 ) however, 

 have discovered series of Ammonites of which one is characterized 

 by their large size and simple mouths, and another consists of small 



(16). Tate and Blake, Yorkshire Lias, p. 263, 1876. 



(17). Munier-Chalmas, Sur la possibilite d'admettre un dimorpbisme sexuel 

 chez les Amnion., Comptcs-Rcndus Sommaircs, Bull. Soc. Geol. France, 1892, 

 p. elxxiv. 



(18). Haug, Etudes s. 1. Amnion. -Juras., Bull. Soc. Geol. France, Vol. XX, 1892. 



