432 MARINE INVERTEBRATES 



ous and much smaller, and are arranged in closely packed radiating 

 rows, the scales becoming obsolete near the uinbones. The mantle of 

 the animal is doubly fringed, and much decorated upon that poi'tion 

 which may be protruded from the shell. The posterior adductor is very 

 large and strong ; the anterior one has almost disappeared. The foot 

 is long and slender, but is evidently a useless appendage, for the ani- 

 mal's habit is stationary. The byssus is large and very strong. Gloves 

 have been made from the byssal fibers of Pinna mixed with silk. The 

 gills are very long and are distinctly not filamentous, the adjacent fila- 

 ments being clearly united by vascular channels. A very curious develop- 

 ment of the osphradium (see page 339) into an erectile process capa- 

 ble of projection beyond the margin of the shell is a unique feature of 

 Pinna. Sometimes thousands of pinnas are torn loose by storms and 

 cast upon the beaches. They do not range north of Hatteras. (Plate 

 LXXIX.) 



GENUS Perna 



P. ephippium. A West Indian shell, also quite common on the east 

 coast of Florida, and especially upon the Keys. It attaches itself by the 

 byssus in great bunches to mangrove roots, sticks, and all manner of 

 rubbish in brackish water. It has a very flat, irregularly shaped shell 

 of a horn-color, and is from two to three inches across. The distinguish- 

 ing feature of Perna is the hinge. It is straight and without teeth, but 

 is marked by a row of transverse grooves to which the ligament is 

 attached. These grooves constitute a row of cartilage-pits. 



FAMILY OSTEEEDJE 



This family probably interests more people than any other one 

 in the class save Amculidce. The latter furnishes the pearl- 

 oysters, the former the edible oysters, the tAvo thus appealing to 

 two very strong human cravings those of vanity and hunger. 



Few realize what an enormous business the oyster trade has 

 become in the United States. The value of it is stated to be over 

 thirteen million dollars annually, twenty-five million bushels of 

 oysters being taken from the Chesapeake alone. The edibility of 

 the oyster has been known from early times, for vast heaps of empty 

 oyster-shells, known as kitchen-middens, occur in various parts 

 of the world. Some of them are of such size and extent as to 

 warrant the belief that their formation must have required cen- 

 turies. Shell-mounds are found along the coasts of Florida and 

 are of some archaeological value. The cultivation of oysters as 

 recorded by Pliny dates from the first century B.C., at which 



