10 INTRODUCTION. 



univalves are said to consist of a single piece, or spiral cone ; 

 but it would be more correct to speak of this piece as forming 

 either the whole or the principal part of the shell : for there is in 

 many instances, a much smaller flattened piece attached to the 

 foot of the animal, which being drawn in when it retires, closes 

 the aperture as with a kind of door, to which in fact the word 

 valve might be very properly applied ; it is called however the 

 operculum, and the little horny plate, frequently drawn out 

 by means of a pin from the aperture of a periwinkle, will 

 present a familiar example. 



The same may be said respecting the bivalves ; for besides 

 the principal portions or valves of which the shell is composed, 

 there are in many species, one or two smaller separate portions, 

 named " accessary plates' by some authors. They are fixed 



by means of cartilages, on 



the back of the hinge. — J\(tJF?5|j^ 



The engraving, fig. 7, < ^^^^^^S^ ) ^ 

 represents the accessary 

 valves of a species of Pholas, Accessary valves of a Pholas. 



which was on this account arranged by Linnaeus with the Mul- 

 tivalves. Nearly allied to the Pholades is a set of shells to 

 which De Blainville has given the name " Tubicola? or inha- 

 bitants of tubes. In this case, the bivalve shell is connected 

 with a testaceous tube or pipe, to which it is attached either 

 by one or by both valves, or in which it lies attached only by 

 the cartilages of the animal. In the genus Aspergillum, the 

 two small valves are soldered into the sides of the tube in such 

 a manner as to constitute a part of it. One of these shells, 

 called the Water-spout, might be taken up by a person not 

 aware of its real nature, and regarded as a pipe or tube prettily 

 fringed, and nothing more ; but upon a closer examination, he 

 would find the two valves, the points of which are visible from 

 the outside of the tube. 



