ortmann: south American naiades. 471 



wc must he satisfied with having pointed out tliis fact, and witii iiinting that i)er- 

 haps further knowledge of the glochidia may furnish at least some criteria for a 

 classification of the species of Diplodon. The same remark holds good of the mar- 

 supial structure. In reference to this we possess a little more knowledge, but it 

 is still too scanty to make an attempt to use it for classification. The genera with 

 the specialized shells of the Castalia- and Hyria-groups are well defined, but their 

 anatomy and their glochidia do not present any remarkable differentiations, e.xcept 

 the structure of the branchial opening in the Cnstalia-group. On the other hand, 

 it appears that the genus Diplodon may finally prove to be composed of an aggrega- 

 tion of more varied forms than heretofore supposed, and that the distinctive char- 

 acters of these are found chiefl}' in the marsupium and the glochidium. 



We may perhaps distinguish within the old genus Diplodon a type, which we 

 might regard as having the more primitive features, such as a marsupium com- 

 posed of interrupted or reticulated septa, occupying the w hole or nearly the w^hole 

 of the inner gill, and probably one of the types of glochidia above described. But 

 it is hard to say, which form this is. I am inclined to regard the hooked glochidium 

 as the more primitive form. The margined glochidium certainly represents a 

 more advanced type. It might finally be possible to split up this genus into smaller 

 genera, one of which should contain these primitive forms, the others to include 

 forms more advanced in regard to their marsupial structure and glochidia. But 

 in the present state of our knowledge this step cannot be taken, and we must be 

 satisfied with an arrangement of the species based upon the characters of the 

 shells. 



The Species of the Hyriin^. 



Genus Diplodon Spix (1827). 



Diplodon Spix, 1827, p. 33 and plate 26.— Diplodon Simpson, 1900, p. 872; 191-1, 



p. 1224." 



Type of genus: — Diplodon ellipticum (cllypticum err. typogr.) Spix, 1827, PL 

 26, figs. 1, 2. (Same type given b\^ Simpson, 1900.) 



The Subdivisions of the genus Diplodon. 



Simpson (1914, p. 1225) has made an attempt to divide this genus into sub- 

 genera and groups. But he also included in it Australian species under the sub- 

 genus Hyriddla Swainson, which we now regard as a separate genus, and an African 



" Diiilodori, as accepted by II. & A. Ailaius (18oS, p. 497) as a siihgeiuis of i'liio, is an entirely 

 hcterogeuous association of species, iuclutliiig forms from all over the world. 



