CROSSOPTERYGII. 69 



occur in the uppermost Devonian of Ohio. Some from the State of New 

 York are perhaps Lower Carboniferous. Characteristic fragments are also 

 known from the Devonian of Belgium. 



Sub-Class 4. Teleostomi. 



The hyostylic fishes commonly named Ganoidei and Teleostei 

 form a continuous series, and are grouped (by Owen) in a 

 sub-class TELEOSTOMI distinguished from the preceding sub- 

 classes by the regular development of dentigerous membrane 

 bones on the margin of the jaw above and below. The skeleton 

 is always more or less ossified, with well-developed membrane 

 bones. The gill-clefts are feebly separated, opening into an ex- 

 ternal cavity covered by a bony operculum. The exoskeleton, 

 when present, consists either of true bone, or of delicate, 

 superposed, calcified lamellae. 



Among the earliest (Lower Devonian) Teleostomi there are 

 representatives of both the great orders, Crossopterygii and 

 Actinopterygii ; but the former considerably predominate, and 

 the latter only become significant in the Carboniferous. Most 

 of the Crossopterygians became extinct before the close of the 

 Mesozoic Era. 



ORDER 1. CROSSOPTERYGII. 



No Teleostomatous fish is known with a more primitive 

 type of paired fin than the lobate form termed "archipterygium" 

 by Gegehbaur. The lowest Order hitherto recognized is there- 

 fore that of the CROSSOPTERYGII (friuge-finned ganoids). This, 

 however, may admit of further subdivision ; for whereas all 

 the Palaeozoic and Mesozoic fishes comprised in it have unibasal 

 paired fins (like those of Ceratodus), the recent Polypterus and 

 Calamoichthys exhibit a tribasal pectoral fin (like that of 

 typical modern Elasmobranchs). The latter type is unknown 

 among extinct Crossopterygians, even those which have the 

 lobate paired fin abbreviated (fig. 23, p. 25) exhibiting a single 

 basal segment of cartilage supporting a shortened, crowded, and 

 partially-fused " archipterygium." 



