5l8 ON CEPHALASPIS AND PTERASPIS 



Amia has scales as thin and flexible as those of a carp, with a. 

 well-ossified skeleton like that of an ordinary Teleostean fish. 



Acipenser and Scapirhynchus have large enamelled dermal plates,, 

 constituting a well-developed exo-skeleton, with a cartilaginous verte- 

 bral column and persistent chorda dorsalis ; 



While, finally, Spatularia, with its mainly cartilaginous endo- 

 skeleton, has a smooth skin, without dermal plates at all. 



In the face of these plain anatomical facts, what is the value of the 

 argument from the development or non-development of the skeleton 

 to the grade of organization of a fish ? 



EXPLANATION OF THE PLATES. 



Plate XIV. [XXXI.]. 



Cepha/aspis. 



Fig. I. Vertical section of the shield of Cephalaspis, magnified loo diameters, a. Outer 



layer, b. Reticular layer, c, d. Middle and innermost substance, e. Vascular 



canals. / Matrix. 

 Fig. 2. Horizontal section of the same, viewed from the outer side, showing the peculiar 



arrangement of the vascular canals along the so-called "sutures," magnified 5a 



diameters. 

 Fig. 3. Thin scale of the inner substance showing the osseous lacunte of two lammce,, 



magnified 200 diameters. 

 Fig. 4. Outline of a vertical section through the shield of Cephalaspis, showing its inflected 



margin (a) and inferior flexible wall (b), magnified 2 diameters. 

 Fig. 5. Section of the inferior wall at the point of transition of the ordinary substance of the 



shield [a] into the thin flexible under layer (b), magnified 100 diameters. 



Plate XV. [XXXII.]. 

 Pteraspis, 



Fig. 1. Vertical section, magnified 100 diameters, a. " Enamer'-ridges forming the outer 

 layer, h. Reticular layer. c, d. Middle and inner substance, c. Cavity filled 

 with matrix— one of the supposed "ossicles." / Vascular canal, g. Matrix. 



Fig. 2. Portion of the shield of Pteraspis Banksii, viewed from within : letters as in fig. i t 

 magnified 10 diameters. 



Fig. 3. Vertical section of inner layer of Pteraspis, showing the lamina and one of the 

 vascular canals, magnified 100 diameters. 



Fig. 4. A flake of the inner layer viewed from within, magnified 25 diameters. «. Vascular 

 canals. 



