PLATE LXI. 



Zone of Harpoceras bifrons. 



Fig. 1. Harpoceras Levisoni, Simp. Young condition with keel, bisulcations, and 



prominent ribs, natural size. Bath Museum. 



2. — — — Side showing the lateral costae, natural size. 



3. — — — Lobe-line of a young costated shell. 



4. — — — Another fragment with Aptychus. Bath Mu- 



seum. 



5. — — — Another specimen, side view of ribbing. 



6. — — — Siphonal area and keel. 



