XVI SYNOPSIS OF 



Metaptera, Raf., emend. Gills attached; eggs in posterior part of gill; a broad expansion of 

 mantle upwards ; superanal opening long ; both siphonal openings fringed. ( Unio alatus, Say.) 



Complanaria, Swain, emend. Gills free, united to the mantle to its margin; outer gill filling 

 entirely with eggs; superanal opening long; both siphonal openings fringed; palpi united along their 

 posterior edge for two-thirds of their length. [Alasmodonta rugosa, Bar.) 



Margaritana, Schum. Gills free from abdominal sac, their posterior extremity not united to the 

 mantle; anal opening or region of mantle not fringed; branchial fringed. [Alasmodonta arcuata, Bar.) 



Alasmodonta, Say, emend. Gills attached to abdominal sac, attached also to the mantle to their 



extremity; branchial siphon fringed; anal large, not fringed; eggs (?). [Alasmodonta margi- 



nata, Say.) 



Strophitus, Raf., emend. Gills attached; eggs received in the whole extent of the outer gill; anal 

 opening not fringed. [Anodonta undulata, Say.) 



Anodonta, Brng. Gills free; eggs throughout the gill; anal opening not fringed. [Anodonta 

 Benedictensis, Lea.) 



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 knowledge of the struc- 

 tural differences of the soft parts of these animals, is not yet sufficiently 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 Anodonta Blainvilliana (nobis), in my 

 Synopsis, 2d ed. p. 31, and I trust that the 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 the existing numerous species — the exo-skeletons 1 (so to call them) of which have 

 only come under our notice — the facilities which a good system ought to afford cannot be 

 reaped by a partial anatomical knowledge, which does not now embrace, probably, one- 

 eighth of the ascertained species of the family. Besides this, I am not at all disposed to 

 think that we can entirely dispense with the aid we find in the various characters of the 

 exo-skeletons, in making our sub-groups. They often, in fact, afford striking and obvious 

 differences, which the eye schooled with but little experience can with -facility and cer- 

 tainty detect, and which, if happily grouped by an experienced eye, may greatly aid the 

 student. These considerations have induced me still to retain nearly the same divisions 

 in this edition which I used in the former one. 



1 Forming the calcareous coverings and fulcra for muscular fibres, as well as protection from exterior forces. 



