2 SIRENOID AND CROSSOPTERYGIAN GANOIDS. 



case. Difficulties of the most grave kind appear whenever it is attempted to frame a 

 definition inclusive of all the fossil forms. Some of the best characters of recent Ganoids 

 are founded upon perishable parts, such as the brain, heart, and intestine. Moreover, 

 palaeontology has already brought to light many intermediate and doubtful forms, which 

 impair almost any precise general statement, while others are likely to occur as the rocks 

 are more diligently searched. Under these circumstances a slow constructive method 

 must be tried, and the desired definition will be attained, if at ' all, at the close and not 

 at the outset of our investigations. 



We are not, however, entirely in the dark as to what constitutes a Ganoid fish. The 

 recent members of the order have come by extinction of connecting forms to constitute a 

 somewhat isolatedj group, capable of definition. The construction of this definition is 

 historically interesting, and in recapitulating the steps by which it was gradually framed 

 we shall be led to explanations which would in any case form necessary prolegomena to a 

 descriptive memoir of the Ganoid order. 



The ichthyological method of Cuvier is well known, and need not be discussed at 

 length. For our purpose it is only necessary to remark that the order Ganoidei was 

 therein broken up into two widely separate sections. The Sturgeons and Polyodons 

 went among the Chondropterygian or Cartilaginous Pishes ; Lepidosteus and Polypterus 

 ranked side by side with the Salmons and Herrings among the Malacopterygii Abdo- 

 minales. Here, too, Protopterus was assigned a place when first included by Owen in 

 the class Pisces. Had Calamoichthys been known to Cuvier it would have gone among 

 the Eels as one of the Malacopterygii Apodes. 



It was a step forward when Agassiz announced his views on the method of classifying 

 fossil fishes. He rescued the Ganoid order as a whole, placing the cartilaginous Sturgeons 

 side by side with the osseous Polypterus and Lepidosteus. But the grounds on which 

 this was done were unsatisfactory. It is, perhaps, almost needless to describe the 

 arrangement of Agassiz. In geological text-books it still holds a respected place, though 

 banished from zoology. We have four groups based upon tegumentary organs—; ganoid, 

 with (usually) rhomboidal or polygonal scales covered with enamel ; placoid, with shagreen 

 or prickly tubercles ; ctenoid, with membranous scales jagged on the hinder edge ; 

 cycloid, with rounded scales destitute of enamel. 



This classification is simple and easy to apply. Modern naturalists are, however, 

 unanimous that the application of any test of texture or form of scales would remove true 

 Ganoids from the group and introduce alien genera having no real affinity with the 

 order. Amia} with its cycloid scales, the two species of Polyodon, with their nearly 



1 Mr. Bridge ('Journal of Anatomy,' vol. xi, p. 621, 1877) considers that on the whole the 

 Teleostean affinities of Amia predominate. This conclusion is not consistent with the views here 

 advocated, but it must not be dogmatically rejected. Amia has not yet found its place in any closely 

 connected chain which includes typical Ganoids ; and while one tenable definition would enable it to be 

 claimed as a Ganoid, another would place it among the Physostomi. This systematic uncertainty, so 



