METAZOA MOLLUSCA. 247 



The tendency .towards becoming an involute shell is 

 seen in Nautilus stenomphalus Sow. (Nos. 593, 594). 

 The involute shell in which the earlier whorls are entirely 

 concealed by the latest whorl is seen in Nautilus pompi- 

 lius Linn. (PI. 595 ; Nos. 596, 597). The development 

 of this species is extremely interesting from a phylo- 

 genetic point of view. The first chamber of the shell of 

 the young Nautilus has a linear scar (PI. 595, fig. 2) 

 which probably marks the spot where the protoconch was 

 broken off. The protoconch itself has never been found 

 in perfect condition in this group, though the scar bears 

 strong evidence of its former existence. There are good 

 reasons why the protoconch of Nautilus should be lost. 

 It was probably made of frail material which would easily 

 be broken while the animal was swimming about in the 

 sea. Then again, when the young, nearly straight shell 

 began to coil, the hard calcareous first whorl would come 

 so close to the delicate protoconch that the latter could 

 hardly fail to be destroyed. The protoconch was doubt- 

 less connected with the first chamber by an 'opening 

 which is plugged up in the present Nautili, the scar mark- 

 ing its position. 



In a section through a young shell of Nautilus pompi- 

 lius Linn. (PI. 595, fig. i), the first living chamber is 

 almost straight, while the rest is slightly curved. This 

 chamber was occupied by the young animal after the pro- 

 toconch stage. It was septa-less like the protoconch and 

 without a siphuncle. In course of time the animal ad- 

 vanced and made a wall or septum behind it. This sep- 

 tum bent downward into the first chamber forming a 

 coecum or the beginning of the siphuncle as seen in the 

 figure. The second septum likewise is prolonged, and its 

 coecum extends downward into the first coecum. the bot- 

 tom of the coecum forming a septum across the siphuncle. 

 This early condition of Nautilus represents the adult Di- 

 phragmoceras with its large septate siphuncle. 



The third septum in Nautilus extends only to the sec- 



