THE FAMILY OF UNIONID^E. XXv'li 



1. Subgenus Iridina, 



Having a granulate dorsal margin, and 1 Non-Symphynote I. exotica. Lam. 



furnished with two siphons. 



O ( 

 -j 



2. Subgenus Spatha, 



Having a dorsal margin non-crenulate, 



and furnished with two siphons. 



-A 



CD 



Jfon-Symphynote I. rubens. Desk. 



3. Subgenus Mycetopus, f TVT o i, -\ir i -e 



I INon-Symphynote Myc. soleniformis. 



Having a straight, smooth, dorsal margin, | 7)' Orb 

 and furnished with a long extensile foot. [ 



After the divisions of Symphynote and Non-Symphynote shells, we have the 

 four conditions in which the outward surface of the shell is found, viz.: 



1. Plicate. 2 4. Sulcate. 5 



2. Nodulous. 3 5. Smooth. 6 



3. Spinous. 4 



Each of these subdivisions group according to the form of their outline, thus: 



1. Quadrate. 7 6. Subrotund. 12 



2. Triangular. 8 7. "Wide. 13 



3. Oblique. 9 8. Obovate. 14 



4. Oval. 10 9. Arcuate. 15 



5. Oblong. 11 



I'Amer. Merid.), was observed by him in the Rio Parana above Corrientes. It resembles an 

 but remains always attached by a bijssus proceeding from the foot. It has anterior and posterior adductor 

 muscles. The interior is not nacree. The inner posterior portion is violet-brown ; the anterior is white. 

 It is attached to rocks below low-water mark. It has not been as well observed in regard to its habits and 

 anatomy as it deserves to be. M. D'Orbigny places it among the Unionidee, in which family it no doubt 

 will be found properly to belong by its anatomical structure. Unfortunately the soft parts were destroyed 

 before an examination was made. The genera Etheria, Mulleria (Acostaea, D'Orb.), Galatea, Cyclas, and 

 Dreissena, also inhabit fresh water, and have their affinities. 



1 Agassiz, in Nomenclator Zoologicus, changes this name to Platyiris as being more correct, but 

 Herrmannseu, in Generum Ulallacozorum, says Platiris is correct. 



2 Unio plicatus. Lesueur. 5 Unio Kraussii. Lea. 



3 Unio pustulosus. Lea. 6 Uuio complauatus. Solan. 

 * Unio spinosus. Lea. 



No regard, of course, is paid in this division to the folds or undulations of the beaks, as all the spe- 

 cies are more or less disposed to this character, which is very valuable in discriminating the species. 

 7 Unio asperrimus. Lea. 12 Unio circulus. Lea. 



6 Unio triangularis. Jianii'.i. " Unio rectus. Lam. 



9 Unio clavus. Lam. " Unio modioliformis. Lea. 



10 Uuio ligamentinus. Lam. 15 Unio crassus. Belzius. 



11 Unio complanatus. Solan. 



