xxii SYNOPSIS OF 



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

 different individuals of the same sptecies, that this condition — freedom and attachment — 

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

 less a generic one. Thus in five large perfect specimens of IT. multiplicaius 1 (nobis), 

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

 for a short distance. 2 In IT. Novi-Eboraci some were attached and some free. IT. 

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

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

 River, JN". 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, jiressus, ligamentinus, laivissimus, verru- 

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

 crenulations on the anal opening, viz.: multiradiatus, 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. 3 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, !N\ Y., had not the 

 anal opening fringed. 



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

 Agassiz, he in Archivfiir Naturgescliichte, in 1852, divides the "]S T ajades" 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. Soiverlyanus, 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. Ilild. and perplexus. Lea. 



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



3. Truncilla. Raf. (Ag.) triangularis. Bar. and arcaformis. Lea. 



4. Lampsilis. Raf. . . . radiatus. 



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



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

 Agassiz: rubiginosus, Eleiniamts, multiplivatus, and subrotundus have all the four leaves of the branchiae 

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



2 See note in Obs. vii. 40. 

 8 Obs. x. 87. 



