G. R. Girty—Triticites. 237 



revolving and then in an axial direction. While as a rule the 

 dark line of the dense layer defines the external surface of the 

 partition as well as of. the revolving wall, the thick inner layer 

 apparently being continuous from one to the 'other, occasion- 

 ally it interrupts the latter and as it assumes an axial direc- 

 tion fans out and either divides, so as to bound both sides 

 of the partition, or spreading, involves it in a nearly uniform 

 dark tint. In this case obscure radial lines, probably of struc- 

 tural origin, can sometimes be made out. The partitions, as 

 we shall shortly see, have otherwise a different structure from 

 the revolving wall. The distribution and behavior of this 

 dark coating seems to me to indicate that it is an original and 

 intrinsic feature of the shell structure, and that it does not repre- 

 sent the contact between two walls nor the plane along which 

 testaceous material was deposited from two sides. Somewhat 

 in contradiction, however, stands the fact that where the walls 

 have become detached the dark layer is often not conspicuously 



Transverse section. Transverse section. Microscopic structure. 



x 15. x 25. x 40. 



retained upon either of them. Figs. 3 and 4, drawn from 

 Triticites secalicus, show the general microscopic structure of 

 the wall, the thin outer dense layer, and the thick inner trans- 

 lucent layer. They show the inner wall as completely surround- 

 ing the interior of the chamber. While not of rare occurrence, 

 it is more common for the inner layer to be developed only on 

 the upper and outer portions of the chamber wall, the lower 

 and inner boundary being formed by the dense outer layer of 

 the volution and chamber preceding. It is evident that the 

 complete inner layer, if it is extended from end to end, would 

 cut off all communication between adjacent chambers (except 

 through the pores). The formation of this layer, therefore, 

 over the back and lower walls is probably due to subsequent 

 deposition. 



Part of the translucent wall is thickly penetrated by opaque 

 rods or tubuli, whose direction is normal to its two faces, and 

 it is this structure which has caused Fiisulina to be described 

 as strongly perforate. These rods or tubes are always consider- 

 ably darker than the translucent wall which they pervade and 

 are as a rule of somewhat lighter tint than the dense outer 

 layer. They usually increase in size inward and taper toward 



