THE FAMILY OF UKIOXID.E. XX111 



0. Eurynia. Raf. . . . rectus. Lam. 



7. Metaptera. Eaf. . . alatus, gracilis, &c. 



8. Alasmodonta. Say. . Alas, marginata. Say. Marg. Raveneliana. Lea. 



9. Obovaria. Raf. . . . retusus. Lam. 



10. Micromya. Ag. . . fabalis. Lea. 



11. Crypogenia. Ag. . . irroratus. Lea. dromas. Lea. 



12. Plagiola. Raf. . . . TJ. securis. Lea. 



13. Ortbonymus. Ag. . . cylindricus. Say. 



14. Tritogonio. Ag. . . tuberculatus. Bar. 



15. Quadrula. Raf. . . . metanever. Raf. 



16. Rotundaria. Raf. . . verrucosus. Bar. 



17. Complanaria. Swain. . Al. complanata, &c. 



18. Pleurobema. Raf. . . TJ. clava. Lam. 



19. Uniopsis. Swain. . . Al. edentula. Say. 



20. Margaritana. Sclmm. . Al. arcuata. Bar. 1 



21. Hemilastena. Raf. . . TJ. debiscens. Say. oriens. Lea. 



22. Unio. Retzius. . . . U. nasutus, &c. 



In these generic divisions, by D'Orbigny, Troschel, and Agassiz — all founded 

 on anatomical structure — it will be observed that they do not differ essentially in 

 their modes of division ; and I do not hesitate to express the opinion that our know- 

 ledge of the structural differences of the soft parts of these animals is not yet suffi- 

 ciently advanced to found a perfect or permanent system. That such a one would 

 be instituted I have not doubted, and I so expressed myself in 1838, in a note on 

 Aiiodoata Blainvilliana (nobis), in my Synopsis, 2d ed. p. 31, 2 and I trust that the 

 surviving able physiologists whom I have cited above, will continue to give their 

 labors to an investigation which cannot but produce rich results ; but I am disposed 

 to think that, until malacologists have examined carefully the soft parts of most of 



1 Prof. Agassiz says that Al. arcuata, Bar. and Marg. margaritifera, Shum. differ, in which I do not 

 agree with him. 



* Aware of the great importance of a knowledge of the soft parts of the species of this family, some of 

 which present very different and well-marked characteristics, I have since the publication of the 2d and 3d 

 editions given much time and laborious attention to their careful dissection and diagnoses, sometimes 

 examining more than a hundred individuals of a species. These descriptions of the soft parts have been 

 published in various papers in the Trans. Am. Ph. Soc. and Jl. Acad. Na. Sci., and in my Observations 

 on the Subgenus Unio, vols. 1, 2, G, T, 8, 9, 10, 11, and 12. These descriptions consist of the Subgenus 

 Unio, 211; Margaritana, 14; Anodonta, 26; Monocondylcca, 2; Mycetopus, 1— in all, the soft parts of 254 

 species. Many embryonic forms, which have also had my particular attention, and which present such 

 diverse and peculiar forms, have been described and figured by me in the Jl. Acad. Na. Sci., vols. iv. and 

 vi., and 06s. v. 6 and 12, so far as I have been able to obtain them. 



Very recently I have received from Dr. F. A. Forel, of Morges, Switzerland, his very able Memoir* 

 on the development of the Unionidse. The anatomy of the embryos, with the illustrations, is worthy of 

 great praise, and cannot fail to be appreciated as an important advance in this direction. 



* Beitriige zur Entwicklungsgescliickte der Najaden. W T urzburg, 18G7. 



