4.22 SUPPLEMENT 
hook. ‘This hook is always perforated by a trench or furrow, 
proportional to its length, and whose edges, in touching, ter- 
minate the canal, which produces at the extremity a hole of 
a form somewhat variable, whence this genus of shells has 
received its name of Terebratula. 
The shell appears to be doubled in extent by a mantle, 
which must be very thin, as Pallas describes it under the 
name of periosteum, As to the body itself of the animal, it 
is contained altogether in a small part of the shell, pressed 
against the summits, and sustained by the double forks, when 
there are any. It appears that the belly is applied upon the 
complex valve, and the back upwards; for it is not placed in 
its shell, as is the body of the bivalves generally, one valve on 
each side. The body of the terebratule is depressed or flatted 
from bottom to top, and perfectly similar on the right and 
left. What constitutes its greatest part is what is now named 
its arm, and what were termed gills by Pallas. The mouth 
is medial, and very large. The intestinal canal, which is 
probably the stomach, of a compressed and conical form, re- 
poses in the angular sac of the plane valve, where it receives 
the esophagus. It is entirely surrounded be a certain black 
matter, which is the liver. 
The gills, or arms, (for authors differ on this point) form a 
considerable fasciculus on each side, almost altogether behind 
the abdominal mass. Each is composed, not only of a single 
triangular ligula, very narrow, pinnate in its entire extent, 
which, attached by its base on the sides of the mouth, would 
turn and be free at its extremity, but also of a ligula longi- 
tudinally pinnate, wider in the middle, and attenuated at the 
extremities. This, attached by one extremity on each side, in 
front of the mouth, after having gone backwards and under- 
neath, reascends upwards, convoluting, directs itself anew 
towards its origin, and, uniting itself in the middle line with 
its fellow of the opposite side, forms a vaulted mass. All its 
