738 CLASS XII. 
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 
Aitheriacea are amongst the preceding families. 
Chama L. (in part), Brue. 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, Pstlopus Pout, comp. characters of family.) 
Sp. Chama Lazarus L., Chama damecornis Lam., Rumen. Amb. Rariteith. 
Tab. 42, fig. 3, D’ARGENVILLE Conchyl. Pl. 20, fig. F, CHEmN. Conchy- 
lienkab. Tab. 51, figs. 507, 508; East Indies ;—COhama macerophylla CHEMN. 
1. 1. Tab. 52, figs. 514, 515, Cuv. R. Ani., éd. ill., Moll. Pl. 95, fig. 2, &c. 
All the species of this family live in the sea. 
Cleidotherus SowERBY. 
Sp. Chama albida Lam., Cleidotherus chamoides Sow., DESHAYES Conchyliol. 
Pl. 29, figs. 1—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., Dusu. Conchyliol. Pl. 28, figs. 4—6, Cuv. R. 
Ant., éd. ill., Moll. Pl. 95, fig. 1; this species was first discovered by 
DEtvo in the chalk strata of Mont Saltre, SaussuRE Voyages dans les Alpes, 
1. Pl. 11. figs. 1—4 (éd. 8vo. Neuchatel, 1803, pp. 277—280). Comp. on 
Diceras LAMaRcK Ann. du Mus. vi. pp. 298—302, and DesHayEs Dict. 
class. d’Hist. nat. v. 1824, pp. 465—467. 
Appendix to Chamacea. 
Family of uncertain position, Rudista or Rudiste. 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 DrsHAYES remarks, 
