738 CLASS xn. 



closed. Foot round, geniculate, not byssiferous. Shell irregular, 

 inequivalve, adhering. Two muscular impressions remote. 



In all the preceding families of the Dimyaria the mantle is 

 entirely open at the anterior part. In the following families the 

 margins of the mantle are united anteriorly. Amongst these the 

 present family is distinguished by its irregular shell, just as the 

 jEtheriacea are amongst the preceding families. 



Chama L. (in part), BEUG. Shell irregular, mostly imbricate 

 or lamellose, with points incurved. Hinge with single oblique 

 tooth in each valve, received in a little pit of the other valve. 

 (Animal, Psilopus POLI, comp. characters of family.) 



Sp. Chama Lazarus L., Chama damcecornis LAM., KUMPH. Ami. Rariteifk. 

 Tab. 42, fig. 3, D'ARGENVILLE Conchyl. PI. 20, fig. F, CHEMN. Conchy- 

 lienkab. Tab. 5 1, figs. 507, 508 ; East Indies ; Chama maccrophylla CHEMN. 

 1. 1. Tab. 52, figs. 514, 515, Cuv. R. Ani., ed. ill., Moll. PI. 95, fig. 2, &c. 

 All the species of this family live in the sea. 



Cleidotherus SOWERBY. 



Sp. Chama alUda LAM., Cleidotherus chamo'ides Sow., DESHAYES Conchyliol. 

 PL 29, figs, i 5 ; from the South Sea at New Holland. 



Diceras LAM. Points large, divaricate, contorted into irregular 

 spires. Cardinal tooth very large, thick, incurved. 



A fossil genus, of which two or three species are known from the upper- 

 most oolite and chalk formations. 



Sp. Diceras arietina LAM., DESH. Conchyliol. PL 28, figs. 4 6, Cuv. R. 

 Ani., e"d. ill., Moll. PI. 95, fig. i ; this species was first discovered by 

 DELUC in the chalk strata of Mont Salere, SAUSSUKE Voyages dans lesAlpes, 

 I. PI. n. figs, i 4 (e"d. 8vo. Neuchatel, 1803, pp. 277 280). Comp. on 

 Diceras LAMAECK Ann. du Mus. vi. pp. 298 302, and DESHAYES Diet, 

 class, d'ffist. not. v. 1824, pp. 465467. 



Appendix to Chamacea. 



Family of uncertain position, Rudista or Rudistcc. Fossils in 

 cretaceous strata. 



Superior valve operculiform, small. Inferior affixed, elongate, 

 often divided internally by septa. 



The fossil conchifers designated by LAMARCK under the unintel- 

 ligible name Rudistes, with which, however, as DESHAYES remarks, 



