PLATE 16. 



Fig. 1. 89. 1. Ormoceras tenuifilum. (Pag. 55.) 



1. A transverse section, showing the edge of an annulation of tlie siphon, and tlie proportionate size of this 

 part of the fossil. ( By a mistake of the artist, tlie upper edge of this ring is far too nearly central to he 

 true.) 



1 a. An artificial longitudinal section of a portion of fig. 1, pi. 1.5, showing the siphon and septa. 



1 b. A longitudinal section in a direction from the ventral to the dorsal side, showing very clearly the position 

 of the siphuncle. The specimen appears contracted at both extremities, which is due to its having been 

 slightly bent ; so that in wearing down, the two extremities are worn beyond the centre ; while the 

 middle portion is central, showing the siphuncle almost in contact with the external shell. 



1 c. A longitudinal section similar to the last, but the shell not bent : the direction of the section is nearly in 

 the ventral and dorsal lines. Septa and outer shell showing double lamins. 



1 d. A longitudinal section, showing the siphuncle apparently central, which is due to a wearing down from 

 the ventral side, and consequently leaving the siphon equidistant from the two lateral margins of the 

 shell. 



) e. A similar section to the last, both of which exhibit more or less distinctly the double lamintc of the outer 

 shell and septa. 



In figures 1 4, c, e, the double laminae of the outer shell and septa are not so clearly distinguishable, 

 the interspaces in the fossil being filled up by calcareous matter, so that both shell and septa appe;u- 

 thickened. In 1 n and 1 rf, particularly the latter, the double lamina> of the septa are q\iite distinctly 

 preserved, the interspaces being empty. 



