98 WEST AMERICAN SHELLS 
pass out. The sinus, as might be supposed is very 
broad and deep. The hinge-teeth are small, while 
the cartilage pit is large and deep. The white shell 
is covered with a thin epidermis. This species 
delights in muddy bays, and is found along the 
whole coast. 
Passing now to a very different group of shells, 
we come first to Thracia curta, Conr.. the Short 
Thracia. In form and markings its shell resem- 
bles Figure 42, though it is somewhat oblong and 
also wrinkled at the rear end of the valves. Liga- 
ment external, hinge-teeth small, sinus shallow, 
length from an inch to two inches. Thracia undu- 
lata, Conr., the Wavy Thracia, also called plicata, 
is a rare species that is larger and thinner than 
the last. 
Figure 86 represents the 
inside of the left valve of 
Periploma planiscula, 
Sby., the Silver Lantern- 
shell,(P.argentaria). This 
is a pretty species, easily 
recognized by its peculiar 
spoon-lke hinge-teeth. Oblong, beaks near the 
posterior end, sinus small, right valve inflated, 
left valve flattened. White, smooth, with fine lines 
of growth, silvery within. An inch or two long; 
Fig. 86 
southern. 
