6 PROCEEDIJSTGS OF THE NATIONAL MUSEUM vol.81 



The foregoing account briefly indicates the uncertainty and dis- 

 agreement that exist among investigators concerning the validity of 

 Megalodiscus. A comparison of Megalodiscus americanus with other 

 North American forms shows a striking resemblance. Thus Diplodis- 

 cus temperatus possesses the small sucker in the center of the acetabu- 

 lum, although it is very inconspicuous in many adult specimens. 

 The acetabulum, although relatively a smaller structure than in 

 Megalodiscus americanus^ is, nevertheless, as wide as or wider than the 

 body. The testes are conspicuously smaller. In other respects the 

 differences are very minor, and as the differences already pointed 

 out are of specific rather than of generic value, there can be little 

 doubt that these two forms are cogeneric. I agree with Chapin 

 (1926) that Megalodiscus ranophilus Millzner is synonymous with 

 Diplodiscus temperatus Stafford. Diplodiscu^ intermedium Hunter 

 seems to be a valid species, in many respects intermediate between the 

 above-mentioned forms. 



Holl (1928a) has described a new species of this group from 

 Triturus viridescens. Apparently the basis for including his form 

 in the genus Opisthodiscus is the presence of a small sucker in the 

 center of the acetabulum and the absence of black concretions in the 

 excretory system. I have already shown that the first character is 

 common to other North American forms of this group, while in my 

 collection there exist examples of Diplodiscus temperatus with con- 

 spicuous granules in the excretory ducts, while other specimens lack 

 these. On the other hand, Holl's description of Opisthodiscus aTneri- 

 canus shows some important differences from 0. diplodiscoides, the 

 type species of Opisthodiscus. The type species lacks an esophageal 

 bulb, the oral sucker and pharyngeal pockets are relatively extremely 

 large, the intestinal ceca are asymmetrical, and the ovary is median 

 and between the testes. Holl's species has none of these characters 

 but is, on the other hand, similar to the other North American forms 

 of the group that have been placed in the genera Diplodiscus and 

 Megalodiscus. It therefore becomes necessary to transfer Holl's 

 species to one of these two genera. Indeed, on comparing some speci- 

 mens of Opisthodiscus ame7ncanus, Avhich Doctor Holl kindly sent 

 me, with some barely mature examples of Diplodiscus temperatus, 

 which were taken from local frogs, I find it impossible to separate 

 the two forms, and I therefore consider Holl's species to be identical 

 with Diplodiscus temperatus. 



It appears, from what has been said above, that all the American 

 species of this group [temperatus Stafford (including ranophilus 

 Millzner and americanus Holl), americanus Chandler, and inter- 

 medium Hunter] are cogeneric. If, however, these species be com- 



