the traces of which are known fronn the lower Cretaceous. The second 

 family of this suborder, Hexostomatidae, is obviously considerably younger, 

 its mennbers have a number of traits indicating considerable secondary 

 complications of a part of the system of the organs along with the reduction 

 of the chitinous elements of the attaching apparatus (see page 421). Inasmuch 

 as it occurs exclusively on the tunas and on the fishes close to them, Cybiidae 

 and Carangidae, one can think that its separation took place not before the 

 Eocene. We suppose that Hexostomatidae descend from Mazocraeidae taking 

 their origin from some sort of Octostoma-like ancestors. This can be sub- 

 stantiated only after a study of the postembryonic development of the 

 corresponding forms inasmuch as at the present time even the young 

 immature individuals of Hexostoma are unknown to us. 



The suborder Discocotylinea is divided into several groups 

 comprising close families; however, as a whole it is sufficiently monolithic 

 and relatively much younger than the preceding one. Discocotylidae, which 

 shows considerable specialization, connected in all probability with the 

 secondary adaptation to fresh waters, is the most separated of all the 

 families of Discocotylinea, (the suborder as a whole was initially marine 

 and only some of its species occasionally penetrate into fresh water, or 

 continually live there with the exception of the present family). In their 

 origin the Discocotylidae are undoubtedly connected with Anthocotylidae 

 from common ancestors from which they are derived. We think that this 

 took place in rather distant times because the working out of the morpho- 

 logical and cyclical peculiarities pertaining to Diplozoonidae could hardly 

 develop in relatively short periods. Taking into consideration the occurrence 

 of Discocotylidae (see page 274 ) and also the probable time of the formation 

 of Anthocotylidae, one can suppose that the former separated approximately 

 in the Paleocene period which is also confirmed by the nature of the 

 distribution of the representatives of the family. 



The 2 families --Anthocotylidae and Diclidophoridae, are very 

 close to the Discocotylidae. Both are encountered on Perciformes as well 

 as on Gadiformes and are connected in their formation with both of these 

 orders as well as with Macruriformes (Diclidophoridae). Undoubtedly, these 

 families have common ancestors and it is even possible that Diclidophoridae 

 represents the lateral branch of Anthocotylidae. The time of separation of 

 both families is linked with the time of appearance of contemporary Perci- 

 formes and even somewhat later, i.e., it is dated in the late Cretaceous 

 or most likely the Paleocene. One can link the appearance of Plectanocotyle , 

 which represents a somewhat isolated branch of the same trunk but secondarily 

 acquiring a certain complication in the structure of the attaching apparatus 

 (see page 428). with practically the same time or somewhat later. 



Two more families --Microcotylidae and Gastrocotylidae probably 

 became separated at the same time or approximately at the same time from 

 the common line of Anthocotyle- -Diclidophoridae on the one hand and 



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