TRANSACTIONS OF WAGNER 

 1396 



TERTIARY FAUNA OF FLORIDA 



replacing the cardinals, while the free valve has an arcuate ridge corrugated 

 on both sides which fits between those of the opposite valve. These are de- 

 veloped by the deposition of shell substance at first on and about the original 

 cardinals, connecting, modifying, or submerging them, but they are soon left 

 behind by the rotation of the valves and hinge. The small posterior laterals 

 are almost always discoverable. In the two millimetres stage the adductor scars 

 are small but distinct, the pallial line broad, entire, and slightly irregular ; there 

 is a distinct escutcheon but no lunule. The young of Chama, unlike Echino- 

 chania, shows an anterior right lateral received into a socket in the margin 

 of the left valve, as in the specimen of C. pellucida, much magnified, which 

 has been figured on Plate liii., Figure i, of this volume. 



The shell of Chama consists of three layers : the outer chalky, frequently 

 brightly colored, and with a partially reticulated tubular structure microscopi- 

 cally. The middle layer is heavier, more glassy, and prismatic, while a thin 

 layer of chalky appearance, vertically tubular, lines most of the shell. The 

 periostracum of many Chamas is fugacious or very tenuous. A few species 

 have a heavy brown cortex, which, in the C. inermis Dall, of the Panama fauna, 

 in some unexplained manner is covered by an outer coating of shelly matter 

 sometimes five millimetres thick, and, where thinner and translucent, allowing 

 the periostracum to be seen through it distinctly. Some other species seem 

 to indicate a similar arrangement, but in the C. inermis the superposition is 

 patently obvious. 



According to Fischer, seventy-five per cent, of the recent species of Chama 

 attach themselves by the left valve ; one species is cited as selecting either valve 

 indifferently. 



In the typical Chama the shell is sessile, very inequivalve, with the free 

 valve (as in all sessile mollusks) flatter, with more or less lamellose or spiny 

 irregular sculpture ; there is no defined lunule ; the ligament is narrow, set in 

 a deep, narrow groove, revolving with the rotation of the valves, the resilium 

 sometimes partly separated and deeply submerged ; the pallial sinus simple, the 

 adductor scars large, subequal, usually rough, the mantle adhering by minute 

 processes which penetrate the tubules of the inner shell layer in some species. 



In the genus Echinochama the shell is nearly equivalve, and sessile in most 

 cases only while young ; the sculpture is radial, spinose, and regular ; a large 

 impressed and conspicuous lunule exists, and also an obscure escutcheon. It 

 is confined to tropical American waters in the recent fauna, while Chama exists 

 all over the world in the warmer waters. 



The name Chama is derived from a Greek word meaning a hiatus or a gaper, 



