ON CEPHALASPIS AND PTERASPIS $17 



Without doubt there is a singularly close resemblance, in the 

 structure of the dermal plates, between Ceplialaspis and Megalichthys 

 — the last being very probably a true Ganoid ; but the point of differ- 

 ence is noteworthy : it is precisely the characteristic ganoin-layer which 

 is absent in Cephalaspis. 



On the other hand, the arrangement of the hard tissues in Pter- 

 aspis reminds one almost as strongly of Ostracion, an undoubted 

 Teleostean. 



The existing fishes to which Cephalaspis presents the nearest 

 resemblance in form, viz. Loricaria and Callichthys, are Siluroid 

 Teleosteans, and not Ganoids ; and, if we take the immediate allies 

 of Cephalaspis and Pteraspis, viz. Coccosteus and Ptericlithys, their 

 analogies with Siluroids, such as Bagriis and Doras, are as strong as 

 those with Acipenser. 



A careful consideration of the facts, then, seems to me to prove 

 only the necessity of suspending one's judgment. That Cephalaspis 

 and Pteraspis are either Ganoids or Teleosteans appears certain ; 

 but to which of these orders they belong, there is no evidence to 

 show. 



If this conclusion is valid, it is clear that the ordinary assumption, 

 that the earliest fishes belonged to low types of organization, falls to 

 the ground, whatever may be the relative estimation in which the 

 different orders of fishes are held. 



But it is said that the great development of the dermal skeleton, 

 combined with the rudimentary condition of the endo-skeleton, shows 

 that these early fishes occupied a low place within their own group. 



Mere a-priori argumentation on such questions as these would be 

 a waste of time ; but, happily, we can put the principle involved in 

 this reasoning to the test by 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. 



