The Chondrostei and Holostei 331 



the possibility of redeposition from older strata must be 

 borne in mind, and so the possible derived nature of Lophi- 

 ostomus affinis that he records {2^g: 207) from the Cam- 

 bridge Greensand — whose organic remains are notoriously 

 derived, according to the opinion of some geologists — must 

 be kept in view. 



But we may next treat together two families which 

 while starting as inhabitants of lakes and rivers in their 

 earlier history — seem gradually to have migrated into 

 marine surroundings, or in some cases to have been more or 

 less anadromous, but which ultimately became thoroughly 

 marine organisms. We refer to the Pycnodontidae and 

 the Pachycormidae. For origin of the former we must 

 look to some ancestral type, probably of Lower Liassic 

 time, that combined inherited characters of the genera 

 Colobodus, Dapedius, Semionotiis and Lepidotus, but which 

 as an evolving family, branched off more and more marked- 

 ly from a common stock during Kimmeridgean, Purbeckian, 

 and Wealden time. Then gradual seaward migration gave 

 rise to species and genera that were typically marine, and 

 some of which survived up to the close of the Eocene, as 

 with Palaeobalistum and Pycnodus. It will be of interest 

 therefore to attempt, as accurately as possible, to trace 

 the geographic and geologic relations of the genera and 

 species, in connection with ecologic changes. 



The oldest genus is Mesodon that starts with M. lias- 

 sicus in the freshwater Lower Lias or "Bucklandi" zone 

 of Central England. In considering it and other "ganoid" 

 fishes, it may here be emphasized again that not a few of 

 the freshwater beds of the Lias, as well as of many other 

 Mesozoic strata have been largely overlooked from the 

 exact descriptive standpoint, owing to the abundant marine 

 organisms found in related strata, and even more owing to 

 the comparative ease with which the often perfect and at- 

 tractive invertebrate marine fossils can be identified, by 

 aid of the elaborate descriptions and figures of these that 

 have been published in European, American and even Asi- 

 atic countries. The most minute details are found in the 

 Memoirs of the British Geological Survey, but here it may 

 be pointed out that over a wide expanse of north-central, 



