PHYLOGENY OF THE PELECYPODA. 



307 



species of the family ; but the dissoconeh has an external layer of prismatic tissue. Cam- 

 era drawings of this specimen show that the prodissoconch is of the same size and form 

 as arc the glochidia when removed from the gills of the parent as shown in the figures, 

 and the same observation holds in regard to the size and form of glochidia and com- 

 pleted prodissoconchs in every species in which I was able to make the comparison. The 

 glochidial shell of Margaritana marginata, Lea, is similar to that of M. undulata. Lea 

 figures the glochidium of five other species of this genus. Of these, three are similar to 

 the glochidium of M. undulata; but two are less triangular and more rounded on the 

 ventral border. A specimen of 3Iargaritana margaratlfera, which is 55 mm. long has 

 the umbos well preserved and they bear at the tip the embryonic shell. As in the pre- 

 vious species the prodissoconch of this species is sharply marked off from the succeeding 

 dissoconeh, and in a camera drawing it coincides in form and size, with the glochidium 

 from the gills of the parent. Specimens of Margaritana deltoldea, 19 mm. long, have 

 the prodissoconch well preserved and it presents the same features in relation to the suc- 

 ceeding dissoconeh as in the above-mentioned genera. The specimens of Margaritana 

 undulata studied were collected in Albany County, New York, and those of M. mar- 

 garitana and M. deltoidea at Ann Arbor, Michigan, by C. E. Beecher. 



Fio. 41. 



Fig. 4.5. 



Fig. 4G. 



Fig. 47. 



Fig. 48. 



Fig. 49. 



FiQ. 44. — IJnio cariosus, £;lochitUum from the gills of the par.-'nt. 



Fig. 45. — Margaritana undulata. glochiduim from the sills of the parent. 



Fig. 46. — Anodon cygnea, glochidium from the gills of the parent. 



Fig. 47. — Anodon flaviatilis, glochidium from the gills of the parent. 



Pig. 48. — Margaritana un'lulata, umbonal tip of a young shell, showing the prodissoconch, p, still in i)lacc and the 

 nepionlc stages of the succeeding dissoconeh. 



Picj. 49. — Anodon fiuviatilis, umbonal tip of a young shell slioiving the prodissoconch, p, still in place and the nepionic 

 stages of the succeeding dissoconeh. 



All the figures (except tig. 46) are of left valves. The arrows are dlrsctsd toward thL' anterior end of the shell ; s, glo- 

 chidial spurs ; h, hinge. All the figures X 50 diam. (Drawn by the author.) 



In Anodon the glochidium is better known than the same stage of other genera of this 

 family, as it has been more often studied by embryologists. Schmidt studied it during the 

 period of parasitic life which it leads on fishes, and good figures of this stage are given by 

 that author. The glochidium of Anodon fluviatills, Lea, fig. 47, is sub-triangular in 



