XVIII 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, hke the upper lobe of the caudal fin, are generally 
provided with fulera. Pelvic fins abdominal. Squamation 
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 
