368 ROBERT TRACY JACKSON ON THE 



outline but is less acutely angular on the ventral border than is this stage of Margaritana. 

 The hinge line is straight; the anterior end of the shell is somewhat the larger and s])i- 

 noiis spurs 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 gi-anulated, the same 

 characters existing in the glochidium of Uiiio and Margaritana. 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 Jlaviatilis I have ob- 

 served in glochidia from the gills of the parent of three other species, namely, jL. nndu- 

 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 gi'eatei* than in the other genera of the family. 



A well preserved specimen of Anodon Jlaviatilis 25 ram. in length, fig. 49, p. 367, has 

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

 ing of this si^ecimen coincides in size and form with the glochidia from the gills of the 

 parent, no increased growth being observed. Tlie larger end of the embryonic shell is 

 directed anteriorly 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 preserved specimens by Lea and others) and presents well 

 defined prismatic structure. The glochidia of Anodon Jluviatilis desci'ibed and the older 

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

 collected in Albany County, ^ew 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 ofthe valves, 

 is convincing proof of the correctness of Schmidt's ol)servation that the larger end ofthe 

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

 from the observations of Balfour and Schmidt that after quitting the gills ofthe pai'cnt 

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

 lead a parasitic 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 Unionidse described ai-e sub-tri- 

 angnlar 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 eqnivalent of what 1 have described as the phylembryonic stage 

 (p. 291) of development in Pclocypods and it may be eompai'ed to the embryos of Mo- 

 diola, Cardium and Montacuta, PI. xxiu, figs. 9-11, also of Ostrea, fig. 25, j). .300, and PI. 

 xxrv, 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; butthis is explained as a peculiarity due to the specialized development of this type. 



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