278 PROCEEDINGS OF THE GEOLOGICAL SOCIETY* [Jan. 6, 



of Pteraspis, the "polygonal ossicles" and the "fibrous bony layer," 

 supposed to be peculiar features of Cephalaspis, have, as I have 

 shown, no existence. Supposing that the shield of Pteraspis, like 

 that of Cephalaspis, covered the animal's head (though there may 

 be some ground for entertaining a doubt on this point), then it may 

 be said that the presence of orbits in one, and their absence in the 

 other, indicates a wide difference between the two genera. It must 

 be remembered, however, that there is precisely the same difference 

 between Pterichthys and Coccosteus, which are admitted by all to 

 be closely allied. 



Though I have had no opportunity of examining the Russian 

 species, I believe I do not err in regarding what Pander describes 

 as the teeth of Cephalaspis as merely an excessive development of 

 the marginal tubercles of the outer layer. It does not appear to me 

 that there is any evidence that the mouth was situated at the margin 

 of the shield ; on the contrary, the inward prolongation of the re- 

 flected ventral layer leads me to suspect that the under surface of 

 the head of Cephalaspis resembled that of Loricaria or of Acipenser. 



Zoological position of Cephalaspis and Pteraspis. 



Leaving for the present Professor Pander's " Conodonts" out of 

 view, Cephalaspis and Pteraspis are among the oldest, if they are 

 not the very oldest, of known fishes ; and it is therefore highly inter- 

 esting to inquire into their position in the scale of ichthyic nature. 



Palaeontologists in general, following Agassiz, classify them as 

 " Ganoids ;" but it is to be feared that few persons who have not 

 paid special attention to recent Ichthyology and to Comparative 

 Anatomy have a clear conception of what is meant by the term 

 " Ganoid." 



The founder of the Order, allowing himself to attach an undue 

 weight to mere secondary characters, included under the head of 

 " Ganoidei" a heterogeneous assemblage of Fishes characterized by 

 very few common characters, save their hard and shining scales, and 

 the abdominal position of their ventral fins, but embracing the Silu- 

 roids, the Gymnodonts, and the Ostracionts, while the genus Amia 

 was allowed to remain among the Clupeidce. 



If these are all Ganoids, and if such are the characters of the 

 Order, then doubtless Pteraspis and Cephalaspis are Ganoids. 



Since the publication of the admirable and philosophica re- 

 searches of Johannes Muller, however, the term Ganoidei has been 

 received in a very different sense by the great mass of naturalists. 

 Muller showed that the great majority of the recent Fishes classed as 

 Ganoid by Agassiz, viz. the Siluroids, the Gymnodonts, the Ostracionts, 

 &c, were in no essential respect different from the Teleostei, or true 

 bony fishes, while the true recent Ganoids formed a small but extremely 

 remarkable assemblage, characterized by a structure in many respects 

 intermediate between that of Teleostei and that of the Elasmobranchii 

 (or what are commonly called cartilaginous fishes). Muller showed, 

 furthermore, that the character of the surface and the histological 

 texture of the scales are of little systematic value, and reduced the 



