'HALOPOD4 



.tult in* I 



tlirtinjjh which the animal ami it> shell piiss from their primitive condition 

 onward, aii-l wi> take the liberty of quoting this passage: "The visceral sac of the 

 animal h;i<l obtained a considerable size. Its form was pointed posteriorly. The 

 mantlt> had secreted a shell of like form. This shell was thu- quite open and of 

 conical shape. It now funned hut a single chamber which was both initial aud 

 habitation chamber, wholly filled by the animal. With the growth of the animal 

 the shell was, naturally, lengthened on the anterior margin. As the animal [shell ! 

 at last became too heavy and had need of a hydrostatic apparatus in order to raise 

 and sink itself, the air-chambers, by which the hydrostatic problem is solved for the 

 tetniliranchiates, were formed. The first of these originated in this way: On one 

 -i>li' * *f the upper portion of the visceral sac a circular and almost enclosed constric- 

 tion was produced. The fold of the mantle thus formed deposited shell-matter 

 making an inclined wall and a division of a part of the originally open initial 

 chamber. The resulting chamber was empty and formed the first air-chamber. 

 . This chamber is, thus, bounded by only one septum and in this case lies behind the 

 w.xll corresponding to the first septum in Nautilus. It therefore corresponds to the 

 initial chamber in that genus. As it here has the same function as the other air- 

 chambers, I have termed it the first air-chamber, although, in fact, it is a remnant 

 of the open initial chamber. Moreover, the second air-chamber is probably formed 

 in part from the anterior portion of the initial chamber. The visceral sac of the 

 animal was now divided by a constriction into an anterior and posterior portion. 

 The anterior portion now forms the actual habitation chamber, but the 

 great visceral sac also fills the posterior portion. The growth of the shell progresses; 

 the shell is again lengthened about the aperture. The animal becomes again too 

 heavy and must form another air-chamber. It loosens itself from the wall of the 

 conch, the visceral sac extends itself along the constriction and the animal moves 

 forward a step in its shell. The mantle surfaces form a new septum and about the 

 elongated portion of the visceral sac the calcareous deposit takes the form of a 

 .sheath or siphonal funnel. Thus originates the siphonal cord of the animal. 

 Hence the sipho of Kndoceras belemnHif'onne must have had its origin in a differenti- 

 ation of the visceral sac." (Op. <-it., pp. 6, 7.) 



The pneseptal cone of .\<inn., must be regarded as a great protoconch expressing 

 in its form the primitive nature of the longicones and brevicones of the tetra- 

 b ranch iates. 



It may be suggested that the solidification of the pra-septal cone may have been 



to some extent due to such .-ccondary causes as have produced the solid guard in 



<nnites. The appearance of these siphones and the crystalline structure of their 



