CHONDROSTEI 485 



very agile .Fish, swimming like a snake, and subsisting on 

 insects and crustaceans. The anal fin is enlarged in the male, 

 and the young are provided with cutaneous gills. Calamichthys 

 may attain a length of nearly 40 cm. 



In the remaining Teleostomi (ACTINOPTERYGII) the paired fins 

 are invariably non-lobate, with abbreviate, multibasal endoskeletal 

 supports. Fin-rays are the main support of both the median 

 and paired fins. Jugular plates are usually replaced by branchio- 

 stegal rays, but both may co-exist. The Actinopterygii are the 

 successors of the Crossopterygii in palaeontological sequence, 

 and when the latter began to decline in Carboniferous and 

 Permian times, the former, mainly represented by the earlier 

 Chondrostei, had already become the dominant Fishes of the 

 period. 



Order II. Chondrostei (Acipenseroidei). 



In these Fishes, the oldest and the most primitive of the 

 Actinopterygii, the fin-rays of the median fins still continue to 

 retain their primitive numerical superiority over the radials, and 

 the tail is heterocercal. There is a single dorsal and an anal 

 fin, which, like the upper lobe of the caudal fin, are generally 

 provided with fulcra. Pelvic fins abdominal. Squaination 

 typically rhombic and ganoid. Vertebral column acentrous. 

 So far as is known the chondrocranium is but little ossified, 

 and the cranial bones are mainly dermal. The secondary pectoral 

 girdle still includes a pair of infra-clavicles. 



The Chondrostei are first represented in the Lower Devonian 

 by the solitary Palaeoniscid genus Cheirolepis, a contemporary of 

 the earliest Crossopterygii. They occur throughout the Mesozoic 

 period, except in the Cretaceous, and also in the Eocene, and 

 while steadily diminishing in number and variety they gradually 

 approximate to their degenerate and in some respects highly 

 specialised descendants, the Sturgeons and Paddle-Fishes of the 

 existing Fish fauna. Of the seven families included in the 

 group the Palaeoniscidae are the oldest and the most generalised. 

 The Platysomidae are a specialised offshoot from the Palaeoniscidae, 

 and, if they are rightly to be considered as Chondrostei, perhaps 

 the same may be said of the problematic Belonorhynchidae. On 



