CONCHIFERA. 



377 



deeply plaited, the outer pair much shorter and very narrow, furnished with a 



free dorsal border, and united behind to each other, and to the mantle : 

 ductors each composed of two elements. 



ad- 



Fig. 184. Right Side. Fig. 185. Leftside. 



Animal of Chama (from Torres Str. Mr. Jukes.) 



Fig. 1S4-. Right side, with the umbonal portion of the mantle removed. 



Fig. 185. Leftside, showing the relative extent of the liver and ovarium. 



a, a, adductors; vi, pallial line; e, excurrent orifice; b, branchial; /, foot and 



pedal orifice; p, posterior pedal muscle; t, palpi; g, gills (contracted); I, liver; o, 



ovarium ; d, dental lobes. 



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

 is laminated by oblique lines of growth, with corrugations at right angles to the 

 laminae ; the foliaceous spines contain reticulated tubuli : the middle layer is 

 opaque white and consists of iU -defined vertical prisms or corrugated struc- 

 ture ; the inner layer, which is translucent and membrauous, 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. 

 185, /, 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. Chama arcinella, which is always attached by the right umbo, has 

 the normal dentition 1:2; it is nearly regular and equivalve, and has a dis- 

 tinct lunule. 



Distr. 50 sp. Tropical seas, especially amongst coral-reefs ; — 50 fms, 

 W. Indies, Canaries, Medit. India, China. 



Fossil, 30 sp. Green-sand — . U. States, Europe. 



Sub-genus f Monopleura; Matheron (= Dipilidia, Math) M. iTnbri- 

 cata, Math. Fig. 187. Neocomian, S. France. Shell attached by the 

 dextral umbo ; valves alike in structm-e and sculpturing ; fixed valve straight, 

 inversely conical, with a long, straight ligamental groove, and obscure hinge- 

 area; opercular valve flat or convex, with an oblique, sub-marginal umbo. 



