]G4 PISCES. 



no single differential character common to all the Ganoids (even iue 

 spiral valve of the intestine is rudimentary in Amia and Lepidosteus). 



The scales from which the name of the order is derived are for 

 the most part of a rhomboidal form, and are always covered with a 

 smooth layer of enamel. They are connected together by articular 

 processes, and encircle the body in obliquely directed rings (fig. 607). 



As regards the structure of the skeleton, the Ganoids are partly 

 cartilaginous and partly bony fishes. Both among the fossil Ganoids 

 and those living at the present time (Sturgeon) there are forms 

 which, by the persistence of the notochord and the formation of bony 

 arches, are allied to the Chimceridce. The cartilaginous cranial 

 capsule is always covered with external membrane bones, and the 

 mandibular suspensorium, the jaws, the branchial arches, and the 

 operculum possess a bony consistency. In the so-called bony 

 Ganoids, the primordial cranium is more or less completely replaced 

 by a bony skull, and the vertebral column gradually becomes bony, 

 inasmuch as the vertebrae acquire, through various intermediate steps, 



FIG. 607. Polypterus bichir. 



the biconcave form of the Teleostean vertebrae, and in Lepidosteus 

 reach a phase of development in which, by the presence of an 

 anterior articulating head, they resemble the opisthoccelous vertebrae 

 of Amphibia. Bony ribs, also, are fairly frequently present. 



The caudal fin is usually heterocercal, and the end of the vertebral 

 column is sometimes continued into its superior lobe; there are, 

 however, forms which are transitional in this respect, and lead to 

 the homocercal (diphycercal) form. The spine-like splints known as 

 fulcra, which are arranged in a single or double row on the upper 

 edge and the first ray of the fins, particularly the caudal fin, are 

 peculiar to Ganoids. (" Every fish with fulcra on the anterior edge 

 of one or more fine is a Ganoid." Joh. Miiller.) 



Anatomically the Ganoids present many points of resemblance to 

 the Selachians. The anterior region of the ventricle is separated off 

 as a rhythmically contractile conus arteriosus, and contains several 

 longitudinal rows of valves, which extend as far as the anterior limit 

 of the muscular investment, and prevent the blood flowing back from 

 the artery into the conus during the diastole. The comb-shaped 



