PLATE VII. 



Fig. 



1. A portion of the test of Scaphaspis Lloydii ; considerably enlarged, to show the form of the surface 



ridges and the coarser structure of the lower layers of the shield. The piece is from near the 

 centre of the shield. Observe the flatness of the ridges. 



2. A similar piece from the shield of Scaphaspis rectus, drawn to the same scale (the lower nacreous and 



cancellated layers not being given). The ridges are of smaller size. 



3. A similar piece from the shield of Pteraspis rostratus. The ridges are much narrower, and are rounded. 



Drawn to the same scale. 



4. A similar piece from the shield of Pteraspis Crouchii. The ridges are nearly as broad as in Sc. Lloydii, 



and quite as flat. Drawn to the same scale. 



5. A portion of the shield of Pt. rostratus, taken nearer to the margin, and including three of the 'pits' 



or mucous-gland-sites. Same scale. 

 .5a. A fragment, showing the cancellated layer or polygonal cavities. Same scale. 



6. A portion of the shield of Sc. Lloydii, from very close to its margin. The ridges become exceedingly 



narrow, but maintain their relative proportion as compared with ridges from similar parts in Pt. 

 rostratus. Compare this with fig. 5, and fig. 1 with fig. 3. 



7. The microscopic structure of the shield of Pt. rostratus. A section has been taken traversing the 



ridges, four of which are seen in section. The dark matter above these is matrix. After Professor 

 Huxley. 



8. Restoration of the cephalic shield of Pteraspis Crouchii, Salter. 



9. „ „ Pteraspis rostratus, Agassiz. 



These drawings and those in PI. VI should be examined with a magnifying glass. The 

 strialions have been very carefully rendered by Mr. Fielding, but it was not possible for him to 

 draw them actually as closely packed as they are in reality. 



10. The microscopic structure of Pteraspis rostratus. A section has in this case been taken parallel 



to the ridges. The vascular tufts and their connections with the polygonal cavities are seen. 

 Original. 



1 1 . Under surface of the rostrum of Pteraspis Crouchii. Restored. 



12. ,, Pteraspis rostratus. Restored. 



13. Ideal longitudinal section through the shield of Pteraspis rostratus. 



14. „ „ Pteraspis Crouchii. 



15. „ „ Scaphaspis Lloydii. 



16. Ideal transverse section passing just in front of the insertion of the spine in Pteraspis Crouchii. 



17. „ „ more posteriorly. 

 19. „ „ more anteriorly. 



18. Transverse section of Scaphaspis Lloydii. 



