gives some basis to suppose the appearance of Dactylogyridae approximately 

 at the same time, also. The third subfamily--L,inguadactylinae, which 

 apparently has a shorter history^ inasmuch as it is linked with Gadiformes 

 to which its ancestors undoubtedly transferred from the Perciformes, forms 

 an exception. Thus, as a whole the family Dactylogyridae represents a 

 relatively young group which attained rather strong development in the post- 

 Paleocene period. As regards the interrelations of the separate genera 

 comprising the families, they are understandable from the preceding text 

 (see pages 346 - 355). 



In conclusion, as we have just analyzed, Dactylogyrinea as a 

 whole represents a complex of families descending either from each other 

 or from very close or from common ancestors of the Dactylogyrus -shaped 

 type. There is no doubt that such also were the ancestors of the second 

 suborder, i.e., Monopisthocotylinea. This suborder has a less mono- 

 lithic character, in the first place because of the fact that into it enters 

 two sharply separated families - -Acanthocotylidae and Microbothriidae. 

 The status of the latter in the system, not only of the suborder, but of the 

 class, is not clear to us (see page 385 ). Because of that they are excluded 

 from further discussion. They can be judged only after special research 

 on the development of their representatives. As regards Acanthocotylidae, 

 just as Diplectanidae in the preceding suborder, in addition to possessing 

 a number of peculiarities of internal organization they often sharply differ 

 by the nature of the development of the attaching apparatus among adult 

 worms, which was indicated in detail above (see page 383). Nevertheless, 

 one can consider the separation of this branch of development from the 

 common trunk of Dactylogyridea and in direct proximity from the beginning 

 of Monopisthocotylinea as most probable, taking into consideration with 

 this the primary nature of Acanthocotylidae parasitizing Elasmobranchii, 



almost exclusively on skates of the family Rajidae. One can think that the 

 formation of this family did not take place before the upper-Cretaceous 

 and most probably during the Paleocene period. 



The four remaining families of Monopisthocotylinea are much P. 457 

 closer to each other, although here also their correlations are not the same. 

 The family of Monocotylidae is closest to the initial dactylogyrid-type. The 

 interrelations within this family were examined in detail in the systematic 

 part (see page 364), but as a whole this family, peculiar exclusively to 

 Elasmobranchii, apparently separated for the first tiine rather long ago. 

 One cannot say with certainty whether the first Monocotylidae appeared on 

 sharks or skates but we can think that this family undoubtedly existed already 

 in the Cretaceous, i. e. , it separated earlier than all the Dactylogyrinea 

 as a whole. The second large family of this suborder, Capsalidae, also 

 examined in detail above (see pages 373 - 383, is genetically very close to 

 Monocotylidae but is considerably younger. In spite of the fact that part 

 of Capsalidae is encountered on very ancient Selachii this undoubtedly is a 

 secondary phenomenon and the development of the family is linked with the 



548 



