334 Evolution and Distribution of Fishes 



So with the passage of time, and with increasing special- 

 ization and condensation of the types in shape, in dentition, 

 in scale-structure, in fin-structure and disposition, the pycno- 

 donts appear wholly to have died out in freshwaters by the 

 Mid-Cretaceous at latest, and to have become a marine 

 series up to the period of the Eocene. 



But some beds, rich in pycnodont remains, deserve criti- 

 cal study. For while classed as marine in the past, practi- 

 cally all of the evidence we have of them, shows that they 

 were at least in part freshwater. Amongst others we may 

 here cite the lowest Cretaceous or Neocomian strata, which 

 with their enclosed organisms, are described by Pictet from 

 the Virgulian rocks of the Jura-system of Neuchatel {241 :- 

 i). From the manner in which the marine invertebrate 

 remains are described with the fishes and the saurians, one 

 might conclude that all are marine. But there are two 

 distinct sets of deposits, the lower evidently marine, the 

 upper freshwater. Writing of the latter he says: "Les 

 beaux fossile sont rares. A la carriere des Brenets, on a 

 recueilli des dents de Pycnodiis, de Sphaerodus, de Stropho- 

 diis, de Lepidotus, et des fragments de carapace de tortue. 

 Mais la piece la plus interessante est le bel echantillon de 

 Lepidotus qui sera decrit plus loin." Then above this bed 

 he describes a calcareous non-fossiliferous dolomite, above 

 which beds "d'eau douce infra-cretace" containing Planor- 

 bis, Valva, Physa, etc. along with Chara remains, are found. 



Similarly the five species of "Pycnodus" from the Neo- 

 comian of Sainte Croix (op. cit. ^G) that are now variously 

 distributed among the recently arranged genera of pycno- 

 donts, are a quite distinct, and evidently freshwater set, 

 as compared with chimaeroids and selachians that are 

 described with them from the Gault and other higher 

 Cretaceous rocks. 



During much of Cretaceous time frequent elevations 

 and depressions of land, as well as silting up of shore tracts 

 were proceeding, and only too often the marine invertebrate 

 remains are listed, or even alone mentioned, while the 

 freshwater and land organisms are neglected. 



But as a consequence of the above change of habitat, 

 the most recent and most highly modified genera, Anomoe- 



