INTRODUCTION. 



39 



at either extremity, and terminates in a kind of point, as in 

 Sanguinolaria Diphos (fig 99 in the plates) ; truncated when 

 it ends in a square or angle, as if cut off; an example of 

 which may be seen in Solen (fig. 60 in the plates). 



Other Bivalves are distinguished as being auriculated, 

 having processes flattened and expanded on either side of the 

 umbones, as in Pecten (cut, fig. 82). When there is one of 

 these on each side of the umbones, it is bi-auriculated ; v/hen 

 only on one side, it is uni- auriculated. When the expansion 

 is very broad, as in Unio alatus (fig. 142 in the plates), and 

 in the Hammer Oyster (cut. fig, 83), the term alated is used. 



Fig. 82, auriculated ; 83, auriculated, alated. 



With regard to these alated species of Uniones, it is neces- 

 sary to observe that they are also " advate," as it is termed ; 

 the two valves being joined to each other by the dorsal edge 

 of the expanded parts, and united so completely in substance 

 with each other, that they cannot be separated without being 

 broken. Many other terms are used to express difference in 

 Bivalves, but being generally applicable to Univalves and 

 Multivalves, as well as to them, they will be found explained 

 at large in the alphabetical part of the work. 



MULTIVALVE SHELLS. 



These are of three different kinds; first, the " dorsal" as 

 they are termed by Linnaeus, because they form a ridge in the 

 back of the animal. They are composed of eight pieces, or 



