PLATE 16. 
Fig. 1. 89. 1. Ormoceras tenuifilum. (Pag. 55.) 
1. A transverse section, showing the edge of an annulation of the siphon, and the proportionate size of this 
part of the fossil. ( By a mistake of the artist, the upper edge of this ring is far too nearly central to be 
true.) 
1 a. An artificial longitudinal section of a portion of fig. 1, pi. 15, 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 lamime. 
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. 
1 e. A similar section to the last, both of which exhibit more or less distinctly the double laminae of the outer 
shell and septa. 
In figures 1 b, 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 appear 
thickened. In 1 a and 1 d, particularly the latter, the double laminae of the septa are quite distinctly 
preserved, the interspaces being empty. 
