302 GEOGKAPHICAL AND GEOLOGICAL DISTRIBUTION. 



(Palseoniscus, Amblypterus), which has its forerunner in the Devo- 

 nian Chirolepis. The primitive heterocereal tail which characterifes 

 the ganoid fishes of the Paleozoic era, and which is stUl borne by 

 many of the rhomb-plated fishes of the early Mesozoic period — 

 Palseoniscus, Ischypterus, Catopterus — is converted by gradual 

 modification into the higher homocereal type, which distinguishes 

 the more important genera of Jurassic fishes (Tetragonolepis, Dape- 

 dius, Lepidotus, and the teleostoid Leptolepis). In Semionotus, 

 which ranges from the Trias into the Lias, the tail is of a well- 

 marked transitional character. Very few of the Jurassic Ganoidei 

 survive into the Cretaceous period, which practically marks the 

 final collapse of this important order of animals, henceforward suc- 

 ceeded by the more highly constituted bony-fishes. The most im- 

 portant remaining group is that of the pycnodonts, or bean-toothed 

 ganoids, whose numerous closely related forms have been referred 

 to several distinct genera, which range collectively from the Car- 

 boniferous period (Platysomus) to the Eocene. Among the Jurassic 

 genera are Microdon, Mesodon, Gyrodus, and Pycnodus, the last, a 

 remarkable example of a persistent type, surviving the close of the 

 Cretaceous period into the Tertiary. Of the group of the cycloid 

 scaled ganoids (Ganoidei cycliferi), represented at the present day 

 by Amia, probably the earliest unequivocal remains are those of the 

 genus Amia itself, which appear in the Cretaceous deposits ; by 

 many naturalists, however, several Paleozoic forms, as Holopty- 

 chius, Asterolepis, Bothriolepis, and Coelacanthus, are referred to 

 this group, and with them also the cffilacanthine Cretaceous genus 

 Macropoma. 



The lung-fishes (Dipnoi), which at the present day are repre- 

 sented by the three very isolated genera Lepidosiren (Brazil), 

 Protopterus (Africa), and Ceratodus (Australia), have left un- 

 doubted traces of their existence as far back as the Permian 

 period, when the genus Ceratodus itself appears (Bohemia, Texas), 

 presenting us with the most remarkable instance of persistence in 

 the whole range of vertebrate animals. Not unlikely the genus 

 may be found to be of still older date, and to have been nearly . 

 contemporaneous with its formidable predecessor, the Diniehthys. 

 Remains of Ceratodus have been found throughout the entire series 

 of Mesozoic deposits, from the Trias to the Cretaceous inclusive. 

 Lepidosiren and Protopterus are not known in a fossil condition. 



