The Chondrostei and Holostei 345 



tralian set of beds. Thus of L. talbragarensis he writes: 

 "This is by far the most abundant fish in the Talbragar de- 

 posits," and again after reviewing other and associated 

 fishes he says : "the preceding genera however are rare, 

 compared with the great shoals of Leptolepis" while struc- 

 turally he draws attention to the fact that In the more 

 primitive species of Leptolepis of the Upper Lias "the 

 vertebral centrum Is a simple constricted cylinder," while 

 in the Kimmerldgean and Purbecklan "the centra are robust 

 and are more or less strengthened by a peripheral ossifica- 

 tion." It would be superfluous at present to attempt to 

 picture the pathway of connection for the Leptolepldae 

 in passing from Europe to Australia. But when the geology 

 and paleobiology of Asis Minor, of Persia, of India and 

 of Africa, are better known, more ample results will doubt- 

 less be forthcoming. 



While Oligopleiirus and Oenoscopus of the Ollgopleurl- 

 dae are from Kimmerldgean and Purbeck beds, and so prob- 

 ably are freshwater, the genus Spathiurus from the Chalk 

 of Mt. Lebanon, and the allied Opsigonus from the Leslna 

 beds of Dalmatia strongly suggest that both are marine 

 derivatives from freshwater Ollgopleuridae which migrated 

 seaward along with so many other types, during early 

 Cretaceous times. An exactly similar claim might be made 

 for Leptolepis neiimayri, also for Thrissops microdon and 

 T. exiguus, from Dalmatian Cretaceous beds. 



Nearly related to Thrissops is a still rare and local genus 

 Lycoptera, of which the species L. middendorfii was secured 

 by Elchwald In the Trans-Baikal region of Siberia. Apart 

 from the nature of the rock, the presence of Estheria mid- 

 dendorfii and remains of Beionorhynchns (p. 324) would 

 stamp the deposit as a freshwater one. The only other 

 species, found In Shantung, China, Is still little known as to 

 environal relations. 



In review then it can be said that all of the genera of 

 isospondylous ganoids originated as freshwater derivations 

 from still older forms of like environment, while one or two 

 derivative genera, and some species of these seem to have 

 branched off as marine fishes, though the evidence for such 

 is by no means wholly convincing as yet. 



