ortmann: south American naiades. 547 



connections. Marmpium of the female locatetl in the inner gill, hut not occupying 

 all of it, leaving about one-fourth or a little less free at the anterior and the posterior 

 end. In younger si:>ecimens the marsupial part is smallei-, but also has a median 

 position. Interlaininar connections of the marsupium forming interrupted septa, 

 without any distinct transverse or (juincuneial arrangement. 



Final Remarks on the Genus Diplodon. 

 It is not claimed that the above arrangement of the species of Diplodon should 

 be regarded as in any sense final. Even the two subgenera, Diplodon and ('ijdomija, 

 cannot be sharply separated: they are intimately connected with each other, 

 and Cydormja is allied with the eUipticus-gvo\i\^ of Diplodon through species like 

 rotundus, foniainianus, and yraius. The other groups cannot be very sharply 

 defined according to shell-characters. It is to be hoped that the anatomy may 

 furnish better criteria for the grouping of the species; but unfortunately not enough 

 species are known from this point of view, and some of those known are not known 

 fully enough. The characters of the glochidia need further special study. The 

 following facts, however, may be emphasized: 



1. In six species of the chilensis-group (frenzeli, imitainr, siniillimus, vicariiis, 

 decipiens, paulista), the marsupium has a distinct tendency to move forward in 

 the inner gill, and in three of these (sirmllimus, vicarius, paidista), it is entirely 

 anterior to the middle of the gill. In five of these sjiecies, the structure of the 

 interlaminar connections is interrupted septiform, or partly- reticulate, but in one 

 species (decipiens) continuous septa are formed. The glochidia of all these species 

 have hooks. 



The tendency of the marsupium to move forward is found in addition only in 

 two species of the ellipticiis-gvou\) {grains and deceptus). 



2. In the charruanus-grou\:) {charruanus, piceus, uniguaijensis, hild(e), the 

 marsupium is rather large and lies in the middle of the gill; sometimes {hildxe) it 

 is smaller; and in two cases {charruanus and hildcv) it has a tendency to move back- 

 ward. In the lacteolus-gYowx) (two species, burrouyhianus and mogymirim), the 

 marsupium is also located slightly more posteriorly. In most of these species, 

 the interlaminar connections form interrupted septa, in part reticulated. But 

 there is one exception, for mogymirim has continuous sejita. This structure is 

 thus only known in two species {mogymirim and decipiens) belonging to different 

 groups. 



3. In the ellipticus-groxiy), the marsupium may be in the middle of the gill 

 {enno), slightly posterior {bertha;), or slightly anterior {gratus, deceptus). Its 



