PLATE 15. 



Fig. 1. S9. 1. Ormoceras tenuifiltjm. (Pag. 55.) 



I. A fragment of this spL-cies, partially covered with the delicate thin outer shell, presenting in the lower 

 part of the figure some obscure markings of the septa ; and above, showing the moderate convexity of 

 a septum. 



1 a. Transverse section of the same specimen, broken through the annulations of the siphon. 



1 i. A portion of the outer shell magnified, showing the fine undulating threadlike lines upon the surface. 



1 c. A longitudinal section of a portion of a laige individual, showing the septa, which are composed of double 

 plates reaching only to the inner lamina of the exterior shell. The distance between the lamina; is 

 unusually great in tlie upper part of tliis specimen, the spaces between tliem being nearly as great as 

 between the septa. 



