470 MEMOIRS OP THE CARNEGIE MUSEUM. 



should bo montionod at this point tluit these hooks appear only in the fully developed 

 glochidiuni, while in younger, immature glochidia they are lacking. 



But it seems that in certain species of Dipludun the hooks are always absent, 

 and in these cases the glochidia fully resemble the figures given by Lea. Of course, 

 in certain cases it is hard to decide whether the glochidia are fully developed, or 

 whether the absence of hooks may be due to the immature condition of the indi- 

 vidual. However, I have a case hi which I believe I am justified in thinking that 

 the glochidia, when fully developed, have no hooks. In three gravid females of 

 D. charruamis, the glochidia w^ere all alike, and no hooks were observed (Fig. 4g). 

 One of these apjmrently was discharging, and thus we should expect matiu'o glochi- 

 dia (unless this were a case of premature discharge). 



In a few other cases I am sure that mature glochidia have no hooks, since 

 they are surrounded around the whole lower edge, with a margin, which possibly 

 represents the first beginning of the permanent shell of the adult (Fig. 4a, i). In 

 other groups of Naiades, with one exception (Anodonta imbecillis Saj' of North 

 America), nothing of the kind is known in glochidia, as long as they remain within 

 the marsupium, and also in these South American forms, when immature, the 

 glochidia do not possess this margin. I never have seen a trace of hooks here. 

 The margin has much the appearance of that formed in young North American 

 Uniomdiv, after the parasitic stage on fish. 



That in these cases the shell should appear at so early a stage, when the larva 

 is still within the marsupium of the mother, is indeed remarkable, and possibly 

 points to the conclusion that the modes and conditions of embryonic develop- 

 ment in the Hyriince differ considerably from those of the Unionidce. 



If the above observations are correct, we would have three types of glochidia 

 in the genus Diplodon: (1) Of triangular shape, with hooks; (2) Of the same shape, 

 without hooks; (3) Of the same shape, and without hooks, but with a margin ai-ound 

 the lower edge, which obliterates the triangular shape. 



Whether the second group is real, or only due to incomplete observation, or 

 passes finally into the third, remains to be seen. At any rate, it is very desirable 

 that close attention should be directed to this question in future work on South 

 American Naiades. 



The size of the glochidia is comparatively large, varying from 0.20 to 0.35 mm., 

 which might be called a good medium size in comparison with the glochidia of the 

 Unionidw. 



1 regret that my observations on the glochidia of the Htjriince are not more 

 satisfactory. There are indications of important differences, but for the present 



