438 MANUAL OF THE MOLLUSCA. 
The shell of Chama consists of three layers; the ezterzal, 
coloured layer is laminated by oblique lines of growth, with cor- 
rugations at right angles to the laminw; 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, J, m). 
Some Chamas are attached indifferently by either valve; when 
fixed by the right valve the dentition is reversed, the left valve 
haying the single tooth. Chama arcinella, which is always 
attached by the right umbo, has the normal dentition 1: 2; it 
is nearly regular and equivalvye, 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, Hurope. 
Sub-genus? Monopleura; Matheron (= Dipilidia, Math.). 
M. imbricata, Math. Fig. 226. Neocomian, Southern France. 
Fig. 225. Bi-radiolites, 3- Fig 226. Monopleura, +. 
p, point of attachment; /, 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 igamental 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 groups, adhering laterally, or rising one 
above the other; the casts of such as are known are quite simple 
and chama-like, 
