PLATE XXI. 



Zone of Arietites obtusus. 



Fig. 1. Arietites obtusus, Sowerby. Side view, natural size. My collection. 



2. — — — Front view, natural size. Do. do. 



3. — — — Another larger specimen, natural size. My 



collection. 



4. — — — Side view, showing the extent of involution. 



5. — — — Lobe-lines, showing the lobes and saddles. 



All the specimens figured in this plate have a portion of the external lamina of the 

 shell preserved, the surface is sculptured with longitudinal and transverse lines, which 

 are finely punctated at their intersections. This kind of sculpture is rare, and found 

 only, as far as I know, in Arietites obtusus, Sow., Ar. stellaris, Sow., and Aegoceras 



PLANICOSTA, SOW. 



