CHONDROSTEI. 167 



The Ganoids reached their greatest development in the Palae- 

 ozoic, Triassic and Jurassic periods. From the Cretaceous 

 epoch onwards they have gradually become less numerous, until 

 at the present day they are represented by a few widely scattered, 

 extremely isolated, and for the most freshwater genera. The 

 Ganoids are here divided into four orders Chondrostei, Cross- 

 opterygii, Lepidostei and Amioidei. 



Order 1. CHONDROSTEI. 



Endo-skeleton largely cartilaginous ; head covered with bony 

 plates. Body naked or with rows of bony plates, or with rhomboid, 

 rarely cycloid scales. Operculum weakly developed, branchio- 

 stegals numerous, few, or absent; teeth small or absent. Infra- 

 clavicle present. Caudal fin usually heterocercal. One dorsal and 

 one anal fin, with fulcra. Paired fins non-lobate. Pelvic fins 

 with a series of basal cartilages. Lower Devonian to present time. 



So far as the facts which can be made out from a study of 

 the extinct forms are concerned, this order agrees with the 

 Crossopterygians in the feebleness of the ossification round the 

 notochordal sheath (except Polypterus), and in the presence of 

 infra- clavicles ; it differs from them in the absence or absorption 

 into the body of the basal lobe of the paired fins, in the 

 heterocercal tail, and in the absence of jugular plates between 

 the rami of the mandible. 



There are two living families, the Acipenseridae and theSpa- 

 tulariidae. The following remarks apply to them : 



The tail is heterocercal. The notochord is persistent and un- 

 constricted. Its sheath is thick, unsegmented, and entirely within 

 the elastica externa. The neural and haemal arches are seg- 

 mented (except in front) and placed upon the sheath, but are not 

 continuous with one another round it (fig. 94). There are inter- 

 calated pieces both in the dorsal and ventral arches, and there is 

 a longitudinal ligament on the neural arches. The haemal arches 

 of the two sides meet ventrally and enclose the dorsal aorta. In 

 addition they carry lateral projections which must be regarded 

 as the transverse processes. These meet ventrally in the tail 

 to enclose the caudal vein, so that in the caudal region there are 

 two canals, one for the dorsal aorta and a ventral one for the 

 caudal vein. The anterior part of the column is continuous 

 with the skull, and here the neural and haemal arches are not 



