PLATE XXIV. 
Zone of the Arietites BucMandi, superior. 
Fig, 1. Aegoceras planicosta, Sow. Specimen with ribs and tubercles. 
2. — — — Smaller shell, showing their growth. 
3. — — — Siphonal area of a small typical shell. 
4. — - — — Do. do. showing growth of spines. 
5. — — — This specimen shows that in middle age the 
spines and ribs become relatively less. 
6. — — — Another shell showing curved spines. 
7. — — — Do. do. excessive growth of do. 
8. — — — Abnormal growth of ribs and tubercles. 
The three last fossils appear to represent the Am. zipJnus, Ziet. All the specimens 
are in my collection, and were collected as types out of a considerable number of fossils 
of the same species at Lyme Regis some years ago, and all are now drawn for the first 
time. 
