PISCES. GANOIDEI. 67 



bony plates as in Scaphirhynchus and Acipenser. (c) Entirely 

 covered with rhomboidal ganoid scales as in Lepidosteus, 

 Polypterus, Palaeoniscus and many extinct forms, (d) Covered 

 with rounded scales shaped like the cycloid scales of Teleosteans 

 as in Amia. (e) Having the trunk and part of the tail covered 

 with rhomboidal scales, and the remainder of the tail with 

 rounded scales as in Trissolepis. 



The teeth also are very variable. The endoskeleton shows 

 every stage of transition from an almost entirely cartilaginous 

 state as in Acipenser to a purely bony state as in Lepidosteus. 

 Sometimes, as in Acipenser, the notochord persists, and its 

 sheath is unsegmented ; sometimes, as in Lepidosteus, there 

 are fully formed vertebrae. The tail may be heterocercal, as 

 in Acipenser, or diphy cereal as in Polypterus. The cartila- 

 ginous cranium is always covered with external membrane 

 bone to a greater or less extent, and the suspensorium is 

 markedly hyostylic. The pectoral girdle is formed of two 

 parts, one endoskeletal and cartilaginous, corresponding with 

 the pectoral girdle of Elasmobranchs, and one exoskeletal 

 and formed of membrane bones, corresponding with the cla- 

 vicular bones of Teleosteans. The pelvic fins are always 

 abdominal. The fins often, as in Polypterus, have spines 

 (fulcra) attached to their anterior borders. 



The order Ganoidei may be divided into three suborders : 



(1) CHONDROSTEI. Living genera Acipenser, Scaphi- 

 rhynchus, Polyodon and Psephurus. 



(2) CROSSOPTERYGII. Living genera Polypterus. and 

 Calamoichthys. 



(3) HOLOSTEI. Living genera Lepidosteus and Amia. 



Suborder (1). CHONDROSTEI. 



The skull is immovably fixed to the vertebral column. 

 By far the greater part of the skeleton is cartilaginous. 

 The notochord is persistent and unconstricted, its sheath is 

 membranous, but cartilaginous neural and haemal arches are 



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