439 "MARINE INVERTEBRATES 
ous and much smaller, and are arranged in closely packed radiating 
rows, the scales becoming obsolete near the umbones. The mantle of 
the animal is doubly fringed, and much decorated upon that portion 
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 OSTREIDE 
This family probably interests more people than any other one 
in the class save Aviculide. The latter furnishes the pearl- 
oysters, the former the edible oysters, the two 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-shels, 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 archxological value. The cultivation of oysters as 
recorded by Pliny dates from the first century B.c., at which 
