XVIII TELEOSTOMI CROSSOPTERYGII 477 



complete, including a pair of infra-clavicles. With rare excep- 

 tions the fin-rays of the median fins retain their numerical pre- 

 ponderance over the supporting radials. The group is divisible 

 into two " sub-orders," the Osteolepida and the Cladistia.^ 



Sub-Order 1. Osteolepida. 



The obtusely or acutely lobate pectoral fins articulate with the 

 pectoral girdle by a single basal endoskeletal element. Nostrils 

 on the ventral surface of the snout. Two dorsal fins and an 

 anal fin. Dermal bones of the ethmoid region often fused with 

 one another and with the premaxillae in front and the frontals 

 behind to form a continuous rostral shield. Infra-dentary bones 

 niay be present. A series of lateral jugular plates often present 

 in addition to the pair of principal plates. The Osteolepida 

 first make their appearance in the Old Eed Sandstone and 

 Devonian formations, where they become abundant. They are 

 also well represented in the Carboniferous, but only one family 

 survived to the Mesozoic period, finally becoming extinct in 

 the Upper Cretaceous. The following are the more important 

 families : — ■ 



Fam. 1. Osteolepidae. — Scales rhombic and thickly en- 

 amelled. Pectoral and pelvic fins obtusely lobate. Tail hetero- 

 cercal. Teeth simple, not complicated by surface infoldings 



Fig. 273. — Restoration of Osteoltins macrolepidota. Old Red Sandstone. 

 (From Traquair.) 



except quite at the base. Genera: — Osteolepis (Fig. 273), 

 Thursius, Diplopterus (Middle Old Eed Sandstone, Scotland), 

 Glyptopovius (Upper Old Eed Sandstone, Scotland), Megalich- 



^ Boulenger, Poissons dii Bassin du Congo, Bruxelles, 1901, p. 2. Smith 

 Woodward {Brit. Mus. Cat. Foss. Fishes, ii. 1891, p. 317 ; and Vert. Palaeont. 

 Cambridge, 1898, p. 78), following Cope, recognises four sub-orders, the Haplistia, 

 Rhipidistia, Aotinistia, and Cladistia. The first sub -order is reserved for the 

 Tarrasiidae, a family which includes only the little known Tarrasius problematicus 

 from the Lower Carboniferous of Scotland. 



