3G8 ROBERT TRACY JACKSON ON THE 



outline but is less acutely angular on the veuti-al border than is this stage of Margaritana. 

 The hinge line is straight; the anterior end of the shell is somewhat the larger and spi- 

 uous spui'S exist at the ventral angle of either valve, PI. xxni, figs. 7-8. A double line 

 surrounds the limits of each valve and the surface of the valves is granulated, the same 

 characters existing in the glochidium of Unio and Margaiitana. The inner line is dis- 

 tinct from the edge of the shell and represents an earlier stage of growth. In A. cygnea, 

 fig. 46, p. 367, the double line represents the thickness of the edges of the shell, and the 

 inner line representing an earlier stage as in fig. 47, is not seen in this view. The glochi- 

 dium of Anodon differs from that of the other genera in the species observed in being of 

 a brown color. A glochidium similar to that described in Anodon Jluviatilis I have ob- 

 served in glochidia from the gills of the parent of three other species, namely, A. undu- 

 lata, A. cylindracea and A. cygnea, fig. 46. There is great constancy in size and 

 appearance of the glochidia in the different species of Anodon observed, the uniformity 

 being greater than in the other genera of the family. 



A well preserved specimen oi Anodon Jluviatilis 25 mm. in length, fig. 49, p. 367, has 

 the embryonic shell or prodissoconch still in place. The prodissoconch in a camera draw- 

 ing of this specimen coincides in size and form with the glochidia from the gills of the 

 parent, no increased gi-owth being observed. The larger end of the embryonic shell is 

 directed anterioily and the hinge line is parallel to the hinge line of the succeeding dis- 

 soconch. The dissoconch has the rugose fluting characteristic of the young (frequently 

 figured at the umbos in well ])reserved specimens by Lea and others) and presents well 

 defined prismatic structure. The glochidia of Anodon Jluviatilis described and the older 

 specimen of the same species as also the glochidia of A. imdulata and A. cylindracea were 

 collected in Albany County, New York, by C. E. Beecher. The glochidia of ^. cygnea 

 are from Belfast, Ireland, and were loaned to me by the Smithsonian Institution. 



The fact that the larger end of the prodissoconch is directed anteriorly in all the species 

 of Margaritana and Anodon in which it was still found in place at the umbos of the valves, 

 is convincing proof of the correctness of Schmidt's observation that the laiger end of the 

 embryo of Anodon is the anterior end. It is a remarkable fact that the completed pro- 

 dissoconch in the various species observed has the same form and size as does the glo- 

 chidial stage of the same species when removed from the gills of the parent. We know 

 from the observations of Balfom- and Schmidt that after quitting the gills of the parent 

 the embryos of Anodon (and probably, from analogy, other genera of the family as well) 

 lead a i)arasitic existence in the gills, fins, etc., of fishes. During this period of parasit- 

 ism considerable anatomical changes take place, yet these changes are not accompanied 

 by any noticeable modification or increase in size of the embryonic shell. 



It is seen that the glochidial shells of the genera of the Unionidae described are sub-tri- 

 angular or subquadrate; but always with a straight hinge line. But a single adductor 

 muscle exists and other anatomical features are in a very early condition of development. 

 This stage is directly the equivalent of what I have described as the phylembryonic stage 

 (p. 291) of development in Pelecypods and it may be compared to the embryos of Mo- 

 diola, Cardium and Montacuta, PI. xxiii, figs. 9-11, also of Ostrea, fig. 25, p. .300, and PI. 

 XXIV, figs. 13-15. The anatomy of the glochidium from the gills of parents is less ad- 

 vanced than is the same (phylembryonic) stage in other genera, as shown in the figures 

 cited; bntthis is explained as a peculiarity due to the si^ecialized development of this tyj^e. 



