328 NEW YORK STATE MUSEUM 



to have yet acquired it, while inversely the phragmocone, as in 

 Atractites, was still so well developed that this genus was at first: 

 unhesitatingly referred to Orthoceras. Where the proostracum is 

 fully developed the animal has discarded the phragmocone entirely 

 as living chamber, and inclosed this former exterior conch within 

 the mantle whereby the rostrum and phragmocone find their position 

 in the posterior end of the animal. 



l"he endosiphocoleon, which externally resembles the proos- 

 tracum, lies within the anterior part of the siphuncle. It is, as we 

 have demonstrated, formed within the endosiphocone. As now the 

 endosiphocone contained the posterior portion of the animal ('' vis- 

 ceral cone" of Bather), and this was inclosed by the mantle, the 

 endosiphocoleon forming at the posterior end of the visceral cone 

 was undoubtedly produced by the mantle and since the sur- 

 rounding endosiphosheath was left behind by the outer mantle,, 

 this more anterior endosiphocoleon is to be considered as 

 secreted within a mantle flap or fold situated at the posterior- 

 end of the animal. Both the endosiphocoleon and proostracum 

 are hence formed in identical places. 



If we further take into account that while in our Proterocamero- 

 ceras a large portion of the siphuncle served as chamber of habi- 

 tation to the animal, and that in the Belemnitidae the animal 

 had entirely withdrawn from the conch, the different position 

 of the endosiphocoleon and of the proostracum relative to the 

 phragmocone will be seen not to constitute a fundamental dis- 

 tinction. One might say that the animal in withdrawing first 

 from the siphuncle and finally also from the living chamber 

 pulled the endosiphocoleon after it till the latter came to lie in 

 front of the old living chamber of the phragmocone. 



It can not be held that the proostracum is a direct further 

 development of the endosiphocoleon in view of the fact that the 

 latter is only found in the early Endoceratidae and could have 

 no place in the later orthoceracones with their shrunken siphun- 

 cles, while, on the other hand the proostracum does not appear 

 till the phragmocone has been reduced to a rudiment in the 

 Belemnitidae. But since the Belemnitidae, as Hyatt has claimed,- 



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