ACTINOPTERYGII. 117 



Oligopleurus. The mandible in this genus is truncated in front and 

 a little prominent ; the teeth are minute. The branchial arches bear a 

 close series of large bony gill-rakers. The vertebral centra are well ossi- 

 fied, and short ribs fit into pits. The first vertebral centrum, articulating 

 with the basioccipital, is composed of two thin discs fused together, but 

 the others are all simple, each bearing its own arch. The dorsal and anal 

 fins are short-based, remote, and directly opposed ; the caudal fin is forked. 

 The typical species, 0. esocinus, occurs in the Lower Kimmeridgian (Litho- 

 graphic Stone) of Cirin, Ain, France; while there is evidence of other 

 species in the Wealden of the Isle of Wight (0. vectensis), and in the 

 Purbeck Beds of Dorsetshire. 



Spathiurus. With greatly extended dorsal fin, and slightly forked 

 caudal. S. dorsalis from the Upper Cretaceous of Hakel, Mount 

 Lebanon. 



The foregoing three families of Isospondyli are exclusively 

 Mesozoic. None of those which still survive date back further 

 than the Cretaceous, while of such the Elopidae and their allies 

 are perhaps the most important. Elops and Albula, as is well- 

 known, are remarkable among " physostomous Teleostei" as 

 exhibiting a rudiment of a spiral valve in the intestine, and 

 traces of a series of valves in the conus arteriosus of the heart, 

 while the former has a well-developed gular plate between the 

 mandibular rami. It is therefore interesting to note that they 

 are widely represented in the Cretaceous and Lower Tertiary 

 periods by numerous closely-related genera. Osmeroides leive- 

 siensis, from the English Chalk, is a typical Elopine with gular 

 plate. Elopopsis from the Cretaceous of Istria, Bohemia, and 

 England, has comparatively large teeth. Albula itself, or at 

 least a fish with identical skull, occurs in the London Clay 

 (Lower Eocene) of the Isle of Sheppey. Chanos is recorded 

 from the Lower Miocene of Chfavon, N. Italy. 



The Osteoglossidae, now not extending further north than 

 Central America in the New World, are represented in the 

 Green River Shales (Eocene) of Wyoming by species of the 

 extinct genus Dapedoglossus. 



Among Isospondyli with small premaxillse but with parietals 

 separated by the supraoccipital, the Clupeidae have of course 

 many extinct representatives. Clupea itself is not certainly 

 known below the Upper Eocene (Monte Bolca. near Verona) ; 

 but a genus apparently differing from it only in the presence 



