ARM-FOOTED MOLLUSKS. Sit 
jaden with their effete particles, and to bring within 
the reach of their prehensile organs the animalcules 
adapted for their sustenance.”* 
Genus TEREBRATULA. 
In this genus, which I select to represent the 
Class, the valves are unequal; and the lower one, 
more prominent than the other, is perforated to 
admit the passage of a short fleshy stem, by which 
the animal is firmly attached to rocks, and other 
foreign bodies. The shell is delicate in texture, 
more or less triangular, and symmetrical. The 
shelly frame-work of the arms, sometimes called 
by collectors the carriage-spring, is attached to the 
inner surface of the upper valve, or that which is 
not perforated. 
Most of the species of this genus, as of the whole 
Class, are known only in a fossil state ; those which 
are recent are widely diffused, and flourish in ex- 
tremely warm and extremely cold climates, as well 
as those which are intermediate. ‘Thus some are 
found in the Indian ocean, beneath the equator ; 
while the 7. psittacea, brought home from the late 
Arctic expedition, was dredged at Boothia in the 
Polar sea. 
* Comp. Anat. i, 279. 
