72 



MOJfOCOUDYKEA. 



VI. SUBGENUS MONOCONDYLCEA. 1 



OVATE. 



*crebristriata. Anih. Blan. 

 Trifjonodon crebri striata. Con. 



OBOVATE. 



Salmeniana. Gould. Blan. 

 Anod. Salweniana. Gould} 



' QUADRATE. 



*rhomboidea. 3 Lea. 



Unio Euphraticusf Bourg. 

 Unio Opjpertif Monge. Bourg. 

 Unio Churchillianus. Bourg. 



Cambodjensis. Petit. 



OVATE. 



Saulcyi. Bourg. Lea. 



1 D'Orbigny, the distinguished traveller in South America, formed the genus Monocondyloea for a 

 group of shells which he first observed, and which possess a single cardinal tooth and no lamellar one. 

 This tooth certaiuly differs from that of the Margaritana, Schum. (Alasmodonta, Say). I am indebted 

 to the great kindness of M. D'Orbigny for his species. The shell figured by Spix, PI. 25, Figs. 1 and 2, 

 under the name of Aplodon inerme, but not described in the text, evidently belongs to D'Orbigny's genus 

 Monocondyloea. It is certainly a most interesting group, and it is to be regretted that we have no 

 description of the soft parts. There seem to be three natural sub-groups in Monocondyloea. 1. Francis- 

 cana, Paraguayana, Parshappii, Gorrientesensis, and Guayrayana have the large tooth in the left valve 

 anterior to the large one in the right valve. 2. Vondembuschiana has the reverse of this, as is also the case 

 with inoscularis, Gould. These two are from the East Indies, while the former are all South American. 

 3. The fossiculifera differs distinctly from both these forms. The dorsal line is not bent into a curve by 

 the teeth, but is indented in each valve, the fosset being fitted by a corresponding projection on the 

 opposed valve. This I propose to call Fossula. When the soft parts of the different species shall be 

 examined, these proposed divisions may be sustained, and the group will consist of Monocondyloea, 

 D'Orbigny, Pseudodon, Gould, and Fossula, Lea. 



2 Dr. Gould described this shell in the Proc. Bost. Soc. Nat. Hist., Dec. 6, 1843, and proposed a sub- 

 genus (Pseudodon) for it. The hinge margin is formed exactly like my Monocondyloea Vondembuschiana, 

 and I have therefore placed it in that genus. 



3 I am disposed to think, judging from M. Bourguignat's figures, that my rhomboidea and his 

 Euphraticus, Opperti, and Churchillianus are all the same. If so, he must have preference. 



4 M. Bourguignat, in Voy. Mer. Mort by Saulcy, describes this shell as having no lateral teeth, and 

 the figure evidently shows it belongs to the genus Monocondyloea. It is from Jaffa. 



6 In a letter from the Abbe* Stabile, Milan, Jan. 18G1, he tells me that Anodonta Uniop>sis, Lam., is 

 only a small thin variety of Alasmodonta Bonellii, Fer. If Uniopsis and Bonellii be the same, then the 

 former has the preference, and the latter is a synonjin. 



