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made to correspond with the description of the genus Terebratula. 
The consideration of the two first of these families we shall therefore 
defer, for a little, and for the present confine ourselves to the exami- 
nation of the Terebratulas only. 
In the shells of this genus, a peculiar cartilaginous and bony con- 
formation exists, serving for the attachment of the animal, and which 
fully warrants their separation from Crania, Calceola, &c. : only slight 
remains of this structure can sometimes be traced in the fossil shells ; 
but in the recent shells it may be seen very distinctly. Plate XYI. 
Fig. is a representation of this part in a recent shell of this genus, 
bearing the general form of A. lacunosa. It is here seen to arise 
from four points of the smaller shell, two just under the hinge, and 
two near to its centre : these, having united by a transverse process, 
again divide, branch upwards, then turn back, and terminate in a 
narrow, transverse, and somewhat circular band. In the longer and 
more oval formed species, having the general figure of A. terebratula, 
Linn, this part is of a somewhat similar shape, but is much smaller, 
and more delicate, as is represented Fig. 6. 
On more particularly examining this part, it will be found still to 
possess some degree of elasticity ; which must, of course, have existed 
in a greater degree during the life of the animal. But it is obvious 
that neither this peculiar construction, nor this elastic state of these 
parts, could have been necessary, merely for the attachment of the 
animal ; and, with the few opportunities which we possess, of examin- 
ing into the habits and economy of this animal, it is difficult to deter- 
mine what was the real office in which its powers were employed. 
Conjecture may, however, be somewhat aided, by attending to the 
directions in which this spring is calculated to act. If its termination 
be drawn perpendicularly upwards, as with the opening of the shell, 
it would, when left to itself, spring downwards ; and again, if drawn 
horizontally towards the beak of the shell, it would gently spring in 
the contrary direction. Hence it may be supposed, that if, contrary 
