ss- 



J3EEKMANT0WN AND CHAZY FORMATIONS OF CHAM PLAIN BASIN 415 



The deposit has an irregular mammillary surface [see pi. 6, fig. i ; 

 pi. 9, fig. 2, 3] ; the larger nodular parts are again composed of 

 smaller, more or less indistinct segments of spherules. Sections 

 demonstrate that it consists of successive layers. These, however, 

 do not lie concentrically around the siphuncle, as one would expect, 

 or fill the chambers by growing progressively from all sides toward 

 the interior but are arranged symmetrically to a diagonal plane, 

 extending in the section [see pi. 9, fig. i, and text fig. 4] 

 from the lower inner corner of the camera to its upper outer corner. 

 Where the cameras had not yet been filled entirely and the mud was 

 able to enter them at the time 

 of the entombment of the shell, 

 the upper inner corners have 

 become filled with it or rarely 

 with anorganic deposits, and 

 the outer lower corners, which 

 also remained empty, with sec- 

 ondary anorganic deposits. 

 Previous to the gerontic stage 

 the cameras became filled with 

 organic deposit to the extent 

 of solidifying the entire shell, 

 and all this deposit arranged 

 itself symmetrically to the diagonal plane of the cameras. 



This mode of deposition can only be understood if the former 

 presence of diagonal membranes is assumed, which served as a 

 base for the deposits. These membranes were of the character of 

 the " pseudosepta " observed by Dewitz, Holm and others. The 

 pseudosepta themselves are not preserved in this species ; they prob- 

 ably were of a similar frail character as the septa themselves which 

 are also lost in a great number of the specimens on account of their 

 great thinness. 



Holm has on the occasion of his description of perfect pseudo- 

 septa and additional structures in the species of Ancistroceras fur- 

 nished a historical sketch of the views of different authors on these 

 membranes and also attempted to give an explanation of their 

 origin.^ From this review we learn that Woodward had first noted 

 the presence of a membrane within the cameras of Ortho- 

 ceras, and that the organic origin of this structure was disputed by 

 Barrande, but reasserted and described more fully from several other 



Fig- 



Vaginoceras oppletum sp. nov. 

 Diagram to show ihe relation of the anorganic 

 deposits to the pseudosepta (/-r) : i'=septa, ic — 

 outer wall of conch, ^j-=septal necks, /z=spaces 

 in cameras free from organic deposit 



iPal. Abhandl. 1886-87. 3:T8ff, 25ff. 



