The Chondrostei and Holostei 327 



towards a higher type of fish than the Chondrostei, to which 

 they technically belong; but the two known genera cannot 

 be the ancestors of this more advanced race, for at least 

 one of its representatives {Acentrophoriis) has already 

 been found in the Upper Permian, while numerous and 

 varied forms are commonly met with in the Trias and 

 Rhaetic. It can only be affirmed that as soon as six im- 

 portant modifications had simultaneously affected the chond- 

 rostean skeleton another vigorous race arose, and a new 

 impetus seems to have been given to variation. These 

 changes comprised ( i ) the almost complete atrophy of the 

 upper caudal lobe, (2) the reduction of the dorsal and 

 anal fin-rays to exactly the same number as their supports, 

 (3) the disappearance of the infraclavicular plates, (4) the 

 loss of the pelvic baseosts, (5) the subdivision of the hinder 

 expansion of the maxilla, and (6) the withdrawal of the 

 preoperculum from its extension over the cheek. 



"Numerous types, in some respects parallel to those 

 already noticed among the Chondrostei, are to be recognized 

 in this later race; and the only difficulty is that, owing to 

 the imperfection of the geological record, very few of these 

 types are revealed until approaching full development. It 

 is true that there are links between most of the families, 

 rendering precise definitions almost impossible; and evi- 

 dences of evolution can be detected in a slight degree, as 

 the different groups are traced upwards in their range. 

 All the families, however, except the modern Lepidosteidae 

 and Amiidae, had already become differentiated before 

 the period of the Lower Oolites." 



The eight families included are the Semionotidae, 

 Macrosemiidae, Pycnodontidae, Eugnathidae, Amiidae, 

 (with a living genus Aviia^ the Bow Fin), Pachycormidae, 

 Aspidorhynchidae, and the Lepidosteidae, (with a living 

 genus Lepidosteiis, the Garpike). 



The Semionotidae includes some twelve genera: Acen- 

 trophoriis, Semionotus, Aphnelepis, Serrolepis, Pristi- 

 somus, Sargodon^ Colobodus, Lepidotus, Dapedhis, Cleith- 

 rolepis, Aetheolepis, and Tetragonolepis. It may be at once 

 said that all were freshwater fishes in origin and habitat, 

 and for reasons that follow. 



