CHAPTER XVIII: THE ROCK OYSTERS 



Family Chamid/^ 



Shell thick, roundish, irregular; valves unequal, ornamented 

 with spines, scales or laminae; beaks sub-spiral; hinge formed by 

 a tooth fitting into a pit; ligament external; mantle closed, gills 

 four, unequal; foot small; siphon small. Large part of family 

 now extinct. 



Genus CHAMA, Linn. 



Characters of the family. Fifty species, attached to coral 

 reefs in tropical seas. Depth fifty to a hundred fathoms. A 

 few species found elsewhere. Forty fossil species in Cretaceous 

 strata, Europe and United States. 



The rock oysters cost all they are worth as an article of 

 diet, for they grow fast to the natural masonry the coral polyp 

 builds, wedged in crevices or attached to stones and shell masses 

 on the ocean floor at considerable depths. They seem to choose 

 most uncomfortable crannies, with no room to grow, and passively 

 allow themselves to be walled in and smothered. Cramped by 

 their habitation, their spines ground down by movable shells, 

 often overgrown with seaweed and encrusted with sediment, all 

 claims to beauty must be abandoned. Yet where they grow in 

 favouring environment some rock oysters have high colouring 

 and elaborate ornamentation of spines. Their parasitic habit 

 costs them a high price. 



The Leafy Chama (C. Lazarus, Linn ) has broad, frond- 

 like spines marking the lines of growth on its shells. It is white, 

 tinted with rose. Each frond is delicately striated. This is 

 found frequently in a perfect state. There is no handsomer 

 Chama. Length, 2 to 3 inches. 



Habitat. — • Mauritius, Philippines. 



The Little Archer Chama (C arcinella, Linn.) is a cool 

 water species, which, when it has a chance, develops a fine array 

 of recurved spines. It is frequently attached to shells of the 



361 



