deration of the two first of these families we shall therefore defer, for a 

 little, and for the present confine ourselves to the examination of the 

 Terebratulae 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 sbells it may be seen very distinctly. Plate XVI. Fig. 7, 

 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 up- 

 wards, then turn back, and terminate in a narrow, transverse, and some- 

 what 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 pos- 

 sess 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 nei- 

 ther 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 examining into the ha- 

 bits and economy of this animal, it is difficult to determine 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 perpendi- 

 cularly 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 to what occurs in other 

 shells, the animal employed some muscular action in opening the shell, 

 on this ceasing, this apparatus would immediately assist in bringing the 

 valves together : and if the animal were attached to any substance by one 



