ON BRACHIOPODA,. 425 
genus may be thus characterized. Body very much com- 
pressed, rounded ; the mantle open in its entire circumference ; 
two tentacular ciliated appendages, as in lingula; shell orbi- 
cular, very much compressed, inequilateral, or a little irre- 
gular, very inequivalve, and without hinge properly so called ; 
one very thin valve, more or less perforated by a fissure ; the 
other patelloid, with a top more or less inclining towards the 
posterior side; four muscular impressions on each valve, two 
of which are larger, and more approximating to the centre ; 
the others more separate, and more anterior. 
These animals live in the northern seas, contained in the 
excavations of rocks, whence they often assume rather an 
irregular form, or they are often rooted by some fibres of the 
central pair of adductor muscles, which traverse a fissure of 
the flat valve. 
