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to what occurs in other shells, the animal employed some muscular 
action in opening the shell, on this ceasing, this apparatus would im- 
mediately assist in bringing the valves together : and if the animal 
were attached to any substance by one end of its cartilage, which runs 
through the beak of the shell, whilst the other end might be connected 
with this apparatus, the resiliency would be increased, and the danger 
lessened of the cartilage, or of the animal itself being injured by those 
shocks which it might receive whilst thus suspended. 
The following are species of this genus which are not, I believe, gene- 
rally known. 
Terebratulites coarctatus. A nearly heptagonal shell, set with beaded 
longitudinal ridges : the larger valve sulcated, with its sides appearing 
as if pinched together, and terminating at the beak with a large fora- 
men ; the hinge small, close, and curved ; and the smaller valve 
convex. Plate XVI. Fig. 5. 
T. ti'iquetrus. A three-cornered shell : the anterior and posterior 
sides compressed ; the superior margin more rounded, but dented in 
the middle : both valves convex, and terminating in a point, with a 
close and curved hinge ; the large valve having a narrow groove, 
commencing in nearly the centre, and extending to an umbilical- 
formed depression in its lower part, just before the beak turns, and 
finishes with a round foramen. 
Two different specimens, both appearing to belong to this species, 
are figured Plate XVI. Fig. 4 and 8. In the former, a small portion of 
the shell still adheres on the lower part of the larger valve, which is 
here shown uppermost ; but the latter specimen, which is here shown 
with the smaller valve uppermost, has none of the shell remaining. 
The substance of these fossils is a very fine close-grained lime-stone. 
I am unacquainted with the habitats of these, or of the former fossil. 
The casts of some species of Terebratulae are of a most extraor- 
dinary form, and obtained a considerable degree of attention from the 
early oryctologists. It appears that these casts were first noticed by 
