348 Evolution and Distribution of Fishes 



trites from the Neocomian of Istria. The writer has al- 

 ready dealt with the Voirons fish-beds; and regarding 

 those of Istria the most suggestive paper is that of J. J. 

 Heckel {2^8 : 201) on the black bituminous chalk beds of 

 the Karst mountain near Goriansk. He there points out 

 that Chirocentrites forms a natural connecting link between 

 ganoid and teleost fishes with decided affinities to the fresh- 

 water ganoid genus Tlirissops. It is unfortunate that he 

 gives no exact stratigraphic or biologic details that would 

 have guided one as to other organisms around, and also 

 as to their habitats. But a somewhat doubtful species of 

 the related Chirocentriis — C. polyodon — has been described 

 from the freshwater tertiary lignites of Sumatra, while the 

 one living species, C. dorab, is marine and found in the 

 Indian and China seas. 



Now the nearest and most primitive ancestors of the 

 above two genera Spathodactyhis and Chirocentrites prob- 

 ably are Leptolepis and Thrissops (Family Leptolepidae), 

 that range from the Upper Lias and Kimmeridgean upward. 

 Ample time therefore elapsed for evolution of the two 

 above teleostean genera. 



It is undoubted however that evolving members of the 

 Chirocentridae must early have passed into a marine Cre- 

 taceous habitat, for in upper Cretaceous beds of Europe, 

 America and even Australia, bulky species of such genera 

 as Ichthyodectes, Portheiis, (Fig 29, p. 227) and Saiirodon 

 must have been as abundant as they were formidable. 



But a consideration at once comes up that has an im- 

 portant bearing on the present inquiry. It is that the 

 Cretaceous, as a geological epoch, is preeminently a marine 

 one, in the beds that have been carefully explored, or that 

 are most accessible to study. But as observed in an earlier 

 chapter, freshwater beds of considerable importance, in- 

 cluding even thick seams of coal, are not at all unfrequent. 



These strata however have been largely neglected, 

 while a study of the teleost fishes associated with them is 

 greatly more difficult than is that of the more easily charac- 

 terized forms of the older strata. We venture confidently 

 to predict that when the vertebrate remains of the Cre- 

 taceous formation are studied, a greatly richer freshwater 



