MON^OCONDYLCEA. 



VI. SUBGENUS MONOCONDYLCEA.' 



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*crebristriata. J.?ii;A. i?/a?i. 

 Trigonodon crehristriata. Con. 



OBOVATE. 



Salmeniana. Gould. Blan. 

 Anod. Saliueniana. Gould? 



' QUADEATE. 



*rliomboidea.^ Lea. 



TJnio Euphraticus? Bourg. 

 JJjiio 0];>pertif Monge. Bourg. 

 Unio Churchillianus. Bourg. 



Cambodjensis. Petit. 



OVATE. 



Saulcyi. Bourg. Lea. 



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OVATE. 



CTn/o Saulcyi. Bourg.* 

 Unio Mechonii. Bourg. 

 Unio Tripolitanus? Bourg. 



OBLONG. 



*Wlieatleyi. Lea. 



SUBROTUND. 



*Franciscana. 3Iori. 

 *lentiformis. Lea. 

 *reticulata. Iloi-i. 

 costulata. Ilori. 



WIDE. 



*Bonellii.' Fer. 



' D'Orbigny, tlie distinguished traveller in South America, formed the genus Monocondylcea for a 

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

 This tooth certainly 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. 2.5, Figs. 1 and 2, 

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

 Monocondylcea. 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 Monocondylcea. 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. Vondenibuschiana 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 Monocondylwa, 

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



" Dr. Gould described this shell in the Proc. Post. Soc. Nat. Hist., Dec. 6, 1843, and jDroposed a sub- 

 genus (Pseudodon) for it. The hinge margin is formed exactl}^ like my Monocondylcea Vondemhuscliiana, 

 and I have therefore placed it in that genus. 



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

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



* M. Bourguignat, in Voy. Mer. Mort hy Saulcj', describes this shell as having no lateral teeth, and 

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



^ In a letter from the Abb^ Stabile, Milan, Jan. 1861, 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, tlaeu the 

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



