XXU STKOPSIS OF 



or absence of "fringes on the siphonal openings." I have frequently observed, in 

 difierent individuals of the same species, that this condition — freedom and attachment — 

 was found. It does not, therefore, seem to be vporthy of a specific difference, much 

 less a generic one. Thus in five large perfect specimens of U. multiplicatus^ (nobis), 

 I found three to be united the whole length of the branchia, while two were opened 

 for a short distance.^ In TI. Novi-Ehoraci some were attached and some free. JJ. 

 Berlandierii has branchiae free and not free. Alasmodonta marginata, as emended 

 by Prof. A. : " Gills attached to abdominal sac." Six specimens from Mohawk 

 River, ^N". Y. I found to be all more or less open. 



As regards the generic character of Unio emended, the "mantle fringed at both 

 siphonal openings." I found the following Uniones without fringe (papillae) on the 

 anal opening, viz.: triangularis, cylindricus, pressits, ligamentinus, IcBvissimus, verru- 

 cosus, metanevra, lacrymosus, dehiscens, and some others. The following have slight 

 crenulations on the anal opening, viz.: muUiradiaius, rectus, retusus, irroratus, ovatus, 

 purpuratus, asper, declivis. The generic character of Anodonta, "anal opening not 

 fringed." The An. Ferussaciana has papillae on the anal opening.^ A. Oregonensis 

 and A. fragilis have crenulations on the anal opening. 



Complanaria, Swain, emended : Alasmodonta rugosa, Bar. Prof. A. says : 

 "both siphonal openings fringed." Specimens from Mohawk, JN^. Y., had not the 

 anal opening fringed. 



Subsequently to the publication by Dr. Stimpson of the MS. diagnoses of Prof. 

 Agassiz, he in Archivfilr Naturgescliichte, in 1852, divides the ^^JSfajades''' into two 

 large groups: — 



First Group has exterior gills joined the whole length of the foot, having no direct communi- 

 cation between the lower free cavity and the upper portion, as in U, iSowerhyanus, triangularis, &c. 



Second Group. Those which have a free communication between the cavity of the gills and the 

 lower cavity of the belly, as in " Unio irroratus, U. lineolatus, Eaf." 



The genera of the First Division are: — 



1. Dysnomia. Ag. . . . foliatus. Hild. and perplexus. Lea. 



2. Scalenaria. Raf. (Ag.) sulcatus. Lea. 



3. Truncilla. Eaf. (Ag.) triangularis. Bar. and arcaforniis. Lea. 



4. Lampsilis. Raf. . . . radiatus. 



5. Canthyria. Swain. . . spinosus. Lea. 



' Two very remarkable characters of some species of Unio seem not to have been noticed by Prof. 

 Agassiz: ruhiginosus, Kleinianus, nmltiplicatus, and subrotundus have all the four leaves of the branchiae 

 pervaded by the uterus, and the first and last have red ova. 



^ See note in Obs. vii. 40. 



» Obs. X. 81. 



