REPORT OF THE STATE PALEONTOLOGIST I903 



329 



can be linked to paleozoic orthoceraconic cephalopods and the 

 latter again quite probably took their origin from endoceratitic 

 forms — by way of Baltoceras — and since therefore there is good 

 reason to consider the Belemnitidae as descendants of the Endo- 

 ceratidae, the similarit}^ of the proostracum and endosiphocoleon 

 is probably more than a mere analogy between unrelated forms 

 due to formation by a like organ (mantle) in the like posterior 

 position, but it partakes more of the nature of the recrudescence 

 of an organ discarded before, when a new use , , 



had been found for it within the same race. 



It does not matter that the endosiphocoleon 

 is a flattened tube and the proostracum only a 

 blade, as a flattened tube would be readily 

 changed into a blade under the stress of a 

 new adaptation. 



8 Endosiphuncular structure of Piloceras 



We have already anticipated the results of 

 our investigation of Piloceras in the synoptic 

 table on page 326, in deriving Piloceras from 

 a more primitive genus Proteropiloceras, that 

 stands on the same plane of phylogenetic 

 development as Proterocameroceras and 

 Nanno. We have also recorded [p. 301] that 

 in Piloceras an endosiphoblade has been observed by Daw^son, which 

 indicates that the endosiphuncular structure may not only be homol- 

 ogous to that of Cameroceras by the possession and strong develop- 

 ment of the endosiphosheaths, but also by the character of the 

 endosiphuncular tubes. 



While, however, in the few specimens of Piloceras in which 

 the apical end has been actually observed, no nepionic bulb 

 has been found, and the siphuncle has been seen to expand 

 gradually and to be inclosed entirely within the phragmocone 

 [see Foord], we have found that P. explanator Whitfield 

 at least retains very distinct traces of the nepionic bulb or apical 

 inflation [see pi. 13, fig.3]. This species points hence clearly 



Fig. 21 Piloceras 

 a m p 1 u m Dawson. Lon- 

 gitudinal section ; showing 

 endosiphocone [j r], last en- 

 dosiphosheath [i'/zj; endosi- 

 photube [e] and remains 

 of endosiphosheaths [r], 

 Dawson's original drawing. 

 (Copy from Foord) 



