ACTINOPTERYGII. 113 



seem to have been equally abundant in Europe during the 

 Eocene and Lower Miocene. The earliest known remains, 

 indeed, were obtained from the Lower Eocene of Reims, 

 France. All the European fossils are merely fragments, such 

 as portions of jaws, vertebral centra, and scales, which differ in 

 no essential respects from the corresponding parts of the recent 

 Lepidosteus. The latest are found in the Lower Miocene of 

 Messel, near Darmstadt. In North America, similar fossils 

 occur from the Eocene upwards, and one small species is known 

 by nearly complete fishes from the Green River Shales of 

 Wyoming. Nearly all of these differ slightly from the recent 

 Lepidosteus in the dentition, and they are hence commonly 

 described under the generic name of Clastes. 



Sub-Order 4. Isospondyli. 



In almost the earliest Mesozoic strata, i.e., in the Upper 

 Triassic, fishes of a remarkably modern aspect begin to appear, 

 only differing from such groups as the herring-tribe in the 

 feeble ossification of the vertebral centra, the presence of 

 minute fin-fulcra, and the possession of ganoid scales. They 

 may be regarded as the forerunners of the ISOSPONDYLI, which 

 exhibit an advance on all the preceding types in the simpli- 

 fication of the mandible. The fishes of this sub-order are 

 characterized as follows: The vertebral centra are more or 

 less calcified, usually complete, and are not known to exhibit 

 even in their most immature stages any indication of separate 

 pleurocentra and hypocentra; none are fused together. The 

 tail is hemi-heterocercal or homocercal. Each ramus of the 

 mandible consists only of two pieces (dentary and angular), 

 rarely with rudiments of a splenial. Infraclavicular plates are 

 absent, and there are not more than four or five basal bones 

 in the pectoral fins. There are no baseosts in the pelvic fins, 

 which are always abdominal. The living members of the sub- 

 order belong to the '* physostomous Teleostei." 



All the Jurassic fishes which seem to belong to this sub- 

 order have a very small premaxilla and the parietal bones in 

 contact ; but in the Chalk there appear certain families in 

 w. 8 



