438 



MAISTFAL OF THE MOLLUSCA. 



The shell of Chama consists of three layers ; the external, 

 coloured layer is laminated by oblique lines of growth, with cop- 

 ■rngations at right angles to the laminae ; the foliaceous spines 

 contain reticulated tubuli ; the middle layer is opaque white, 

 and consists of ill-defined vertical prisms or corrugated struc- 

 ture ; the inner layer, which is translucent and membranous, 

 is penetrated by scattered vertical tubuli; the minute processes 

 that occupy the tubuli give to the mantle (and to the casts of the 

 shell) a granular appearance (Fig. 224, I, m). 



Some Chamas are attached indifferently by either valve ; when 

 fixed by the right valve the dentition is reversed, the left valve 

 having the single tooth. Ghama arcinella, which is always 

 attached by the right umbo, has the normal dentition 1:2; it 

 is nearly regular and equivalve, and has a distinct lunule. 



Distribution, 50 species. Tropical seas, especially amongst 

 coral reefs; — 50 fathoms. West Indies, Canaries, Mediterranean, 

 India, China. 



Fossil, 40 species. Green-sand — . United States, Europe. 



Sub-genus? Monopleura ; Matheron (= Dipilidia, Math.). 

 M. imbricafa, Math. Eig. 226. Neocomian, Southern France. 



Fig. 225. Bi-radiolites, -|- Fig 226. Monopleura, -|-. 



p, point of attachment ; I, ligamental groove ; a, a, corresponding areas. 



Shell attached by the dextral umbo ; valves alike in structure 

 and sculpturing ; fived valve straight, inversely conical, with a 

 long, straight ligamental groove, and obscure hinge-area ; oper- 

 cular valve flat or convex, with an oblique, sub-marginal umbo. 

 Fossil, 10 species. Neocomian — Chalk. France, Texas. They 

 are commonly found in grpups, adhering laterally, or rising one 

 above the other ; the casts of such as are known are quite simple 

 3,nd chama-like.. 



