320 ^ NEW YORK STATE MUSEUM 



served principally as the receptacle for the generative organs, which 

 in Nautilus are situated in the posterior part of the visceral sac. 



Hyatt determines the close of the nepionic age in N a n n o 

 a u 1 e m a with the formation of the first endosiphosheath, after 

 which in that form the endosiphotube becomes plugged and thus 

 the open connection closed with the embryo bag or if the latter 

 had been already destroyed, that with the outside. We have no 

 evidence that such a process took place in C. brainerdi 

 after the formation of the first endosiphosheath though here also 

 the matrix did not enter deeper from the outside into the endo- 

 siphotube than the thickness of one or a few endosiphosheaths, 

 but it seems to us that the nepionic stage could not be well con- 

 sidered as ended till the nepionic bulb or preseptal cone had been 

 entirely left by the visceral sac of the animal or, in other words, 

 had become filled with endosiphosheaths. 



The tube passing through this first endosiphosheath is still 

 both endosiphotube and endosiphocoleon, the differentiation be- 

 tween these two not yet having taken place. Where and when 

 they become differentiated I am not prepared to say. But this 

 differentiation is clearly consequent on the widening of the 

 siphuncle. The latter, as nepionic bulb has only a diameter of 

 2 mm at the perforation of the first endosiphosheath ; it increases 

 to about 10 mm where the formation of the septa begins, meas- 

 ures 15 mm where the endosiphocoleon is fully developed [pl.7, 

 fig. 10] and 20 to 25 mm at its passage into the living chamber of a. 

 mature individual. With the increase of the diameter of the 

 siphuncle that of the major diameter of the endosiphocoleon 

 apparently keeps pace. Since, however, as the animal removes itself 

 more and more from the nepionic conch, only a narrow fleshy band 

 is left behind, a new narrow tube is secreted by the latter within this 

 older endosiphocoleon, as we have shown above [see pl.7, fig.2 and 

 text fig.8]. This is the endosiphotube. As we have indicated in 

 text figure 13, no differentiation between these tubes has yet taken 

 place near the apex. If we take the long slender nature of the 

 apical conch in account, it appears quite probable that the two tubes 



