Sl6 ON CEPHALASPIS AND PTERASPIS 



" 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 philosophical researches 

 of Johannes Muller, however, the term Ganoidei has been received in 

 a very different sense by the great mass of naturalists. MiJiler 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 remark- 

 able assemblage, characterized by a structure in many respects inter- 

 mediate between that of Teleostei and that of the Elasmobrancliii (or 

 what are commonly called cartilaginous fishes). Muller showedj 

 furthermore, that the character of the surface and the histological 

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

 diagnostic marks of a Ganoid, visible in the external skeleton, to two 

 — the presence of " fulcra " and the articulation of the scales by 

 gomphosis. The rest of the essential characters of the Ganoids are 

 entirely derived from the soft parts — the brain, the heart, the branchia;, 

 and the air-bladder. A Ganoid is in fact distinguished from any other 

 fish by the following peculiarities. 



The optic nerves form a chiasma ; the bulbus aortae is rhythmically 

 contractile, and provided with several series of valves ; the branchiae 

 are free ; there is an air-bladder connected by an open duct with the 

 intestine ; the ventral fins are abdominal. These essential characters 

 are shared by only six genera of existing fishes — Lepidosteus, Polyp- 

 terns, Amia, Acipenser, Scapirhjnclius, and Spatidaria — which are no 

 less singular in their distribution than in their anatomy. All are 

 essentially freshwater fishes ; all are found in the northern hemisphere ; 

 three — Lepidosteus, Amia, and Spatularia — are exclusively North 

 American ; Polypterus is only known in the Nile, while Acipenser is. 

 common to Europe, Asia, and North America. 



Now what evidence have we that either Cephalaspis or Pteraspis 

 are in the proper sense Ganoids ? There is nothing about their dermal 

 covering peculiarly characteristic of Ganoids ; and as to the rudiment- 

 ary state of ossification of the vertebral column, there are Teleostean 

 fishes (e.g. HelmicJithys) quite as imperfect in this respect as any 

 Ganoid. 



