into one order- -Clupeiformes in contrast to the opinion of the majority 

 of the ichthyologists who separate esocid fishes into a special order of 

 Esocoiformes (Berg, 1940; Nikolsky, 1954), One can consider it as 

 commonly accepted that Esocoidei, which arose at the end of the 

 Cretaceous period, descend from some sort of Salmonoidei (possibly 

 from Osmeridae). Consequently the parasitizing of Tetraonchus on 

 both groups independently has a phylogenetically determined character 

 and underscores their indubitable consanguinity; and, it seems to us, 

 forces us to consider the point of view of L. C. Berg as more correct. 

 At the same time, we can suppose that the transfer of Tetraonchus to 

 parasitizing Esocoidei is a very old phenomenon because the divergence 

 of contemporary Salnnonidae and Esocidae is very great both morpho- 

 logically and biologically. It seems quite probable to us that this 

 transfer took place in the period of the formation of Esocidae, that is-- 

 not later than the Oligocene. 



As a whole the fauna of monogenetic trennatodes of Clupei- 

 formes is very heterogeneous and consists of different groups in origin 

 and time of adaptation to present hosts. At that same time we can con- 

 sider that the phylogenetically primary parasites of the Clupeiformes 

 are, on the one hand, the marine forms --Mazocraeidae, and on the 

 other--the fresh water Tetraonchidae. It is most probable that the p. 309 



first family had its origin in the Cretaceous period; whereas the 

 second considerably later, from the Eocene or perhaps Paleocene 

 period. 



Up to the present time the actual number of monogenetic 

 trematodes on Scopeliformes is unknown with the exception of one 

 species --Ancyrocephalus alatus Chauhan which, as was indicated 

 above, was probably accidentally discovered on these fishes (page 222), 

 Even if one recognizes its origin as normal (natural, regular, nobis) 

 then it, without any doubt, chanced upon the present fishes secondarily and 

 rather recently, for its basic hosts are fishes of different orders. 



The fauna of Monogenoidea of cyprinifornn fishes is very 

 considerable in its composition. It is composed of representatives of 

 three orders pertaining to both subclasses. 



As is known, the Cypriniformes are basically freshwater 

 fishes. They are divided into two groups which are accepted by Berg, 

 who, however, indicates that it is probable that they represent two 

 separate orders, Cypriniformes (s. str. ) and Siluriformes, The time 

 of the appearance of these two groups, connected to each other by the 

 presence of the Webberian organ, coincides apparently with the be- 

 ginning of the Cretaceous period (Nikolsky, 1954). 



358 



