REPORT OF THE STATE PALEONTOLOGIST I903 



327 



7 Similarity between the endosiphocoleon and the proostracum of 



belemnites 



An inspection of the system of surface lines of the endo- 

 siphocoleon consisting of forward arching transverse ridges and 

 longitudinal lines can not fail to suggest the proostracum of the 

 belemnites; and a study of the relative position of the two organs 

 and of the probable phylogenetic relations of 

 the Belemnitidae with the Endoceratidae 

 makes this comparison seem less farfetched 

 or strained than would appear at first 

 glance. 



The belemnite shell, wdien complete, con- 

 sists, as is well known, of three parts [see 

 text fig.2o]. These are the rostrum, the 

 phragmocone and the proostracum. Of 

 these the rostrum or guard is a later acquisi- 

 tion which does not concern us here. The 

 phragmocone is identical with the phrag- 

 mocone of the early cephalopods which here 

 however has become entirely inclosed 

 within the mantle. From the dorsal side 

 of the last large chamber of the phrag- 

 mocone (the former living chamber of the 

 conch) proceeds a broad, thin, somewhat 

 arched blade, the proostracum, which con- 

 sists of two stronger longitudinally striated 

 lateral regions -and a very thin intercalated 

 dorsal blade. In the typical belemnites this 

 organ has a size much surpassing that of 

 the rostrum and phragmocone as in the 

 restoration here copied ; and in later forms both the latter organs 

 become reduced,"^ while, on the other hand, if the Belemnitidae are 

 traced backward in geologic history, the proostracum becomes 

 smaller and more insignificant and the Triassic forms do not seem 



¥ig. 20 Restoration of a Be- 

 lemnite shell : H, rostrum ; /%, 

 Phragmocone ; Po, Proostra- 

 cum. (.Copy from Zittel) 



iThe homologies of the different parts of the cuttlebone or sepion of the 

 Sepia with those of the belemnite shell are not yet clearly estahlished as the 

 ■differing views of Bather [1888, p.298] and Blake [1888, p.376] evince. 



