VERTEBRATA 393 



of which continue prominent in the Devonian. The most impor- 

 tant genera are Atrypa, Spirifera (III, 7), Pentamerns, and AVy/2- 

 chotreta (III, 8). 



Mollusca. — The Bivalves show no very significant changes 

 from the Ordovician, but the Gastropods, especially such forms 

 as Capulus (PL III, Fig. 11) increase decidedly; other well 

 represented genera of these shells are Platyo stoma (III, 9) and 

 Cyclonema (III, 10). Pteropods are smaller and less numer- 

 ous than before ; a very common form is the little nail-shaped 

 shell, Tentaculites, which is doubtfully referred to this group, but 

 may belong to the Worms. Among the Cephalopods Orthoceras 

 (III, 12) continues abundant, but Lituites has grown less common 

 and Endoceras has disappeared, while coiled shells like Trochoceras 

 (III, 14) are not infrequent. The shells with curiously contracted 

 mouth openings, like Phragmoceras (III, 13) are more commonly 

 found than in the Ordovician. 



Vertebrata. — The remains of Ostracoderms and Sharks show 

 that Vertebrates certainly existed in the Silurian, but the known 

 remains are so fragmentary that a description of these curious 

 fishes and fish-like animals will be reserved for the following 

 chapter. 



Explanation of Plate III, p. 392. 1. Astylospongia praemorsa. 2. Grapto- 

 lithes clintonensis. 3. Favosites Forbesi. 4. Lepadocrinus Gebhardi. 5. Lingu- 

 lella cuneata. 6. Orthis elegantula. 7. Spirifera crispa. 8. Rhynchotreta cuneata. 

 9. Platyostoma niagarense. 10. Cyclonema cancellatum. 11. Capulus angulatus. 

 12. Orthoceras annulatum, 1/2. 13. Phragmoceras parvum, 1/2. 14. Trochoceras 

 desplainense, 1/2. (Fig. 1 after Roemer, Figs. 2-14 after Hall.) 



