280 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Jan. 6, 



this reasoning to the test hy direct observation. This principle 

 clearly is, that the development of the exo- and endo-skeletons stands 

 in some ratio to the general perfection of the organization of a fish. 



Now the existing genera of Ganoids are, as I have said above, 

 characterized by certain anatomical peculiarities common to all ; 

 and, in every essential of organization, no one can be said to be 

 superior or inferior to another. The same kind of brain, heart, and 

 respiratory organs are to be found in all ; nevertheless, Nature seems 

 to have amused herself with working out in this small group every 

 possible variety and combination of endo-skeleton and exo-skeleton. 



Lepidosteus has a greatly developed exo-skeleton, and the most 

 Salamandroid vertebra known among fishes. 



Polypterus has an equally well-developed exo-skeleton, and a well- 

 ossified but piscine vertebral column. 



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 ver- 

 tebral 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 skele- 

 ton to the grade of organization of a fish ? 



EXPLANATION OF THE PLATES. 

 Plate XIV. 



Cephalaspis. 



Fig. 1. Vertical section of the shield of Cephalaspis, magnified 100 diameters. 

 a. Outer layer, b. Reticular layer, c, d. Middle and innermost sub- 

 stance, e. Vascular canals, f. 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 " su- 

 tures," magnified 50 diameters. 



Fig. 3. Thin scale of the inner substance showing the osseous lacunae of two 

 laminae, 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 sub- 

 stance of the shield (a) into the thin flexible under layer (b), magnified 

 100 diameters. 



Plate XV. 

 Pteraspis. 



Fig. 1. Vertical section, magnified 100 diameters, a. " Enamel "-ridges forming 

 the outer layer, b. Reticular layer, c, d. Middle and inner substance. 

 e. Cavity filled with matrix — one of the supposed " ossicles." f. Vascular 

 canal, g. Matrix. 



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

 in fig. 1 : magnified 10 diameters. 



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

 of the vascular canals, magnified 100 diameters. 



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

 a. Vascular canals. 



