MULTIVALVES AND BIVALVES. , 37 



indented and the animal is not siphonless, though the siphons are 

 not very long. Of these there are only sixteen, one of which 

 Cardiumha.s bold radial ribs; one Isocardia has twisted beaks ; 

 and one Lepton is oblong with almost square corners. Let us put 

 this into form : 



Grooves radial Cardium. 

 Grooves concentric 



Beaks twisted Isocardia. 

 Beaks not twisted 



Hinge without teeth Galeomma. 

 Hinge with teeth 



Shell oblong Lepton. 

 Shell triangular 



Two teeth in each valve A start e. 

 Three teeth in each valve Circe. 



Here, Astarte the more important genus can be further dis- 

 tinguished by its having all the teeth entire, while Circe has one tooth 

 cloven, so as to make the three look like four. We can continue 

 this table : 



Shell circular or sub-orbicular 

 Ligament internal 



Anterior scar long and extending within pallial line 



Loripes. 



Anterior scar orbicular Montacitta. 

 Anterior scar oval 



Left cardinal small Lasaa. 



Left cardinal thick and erect Kellia. 



In this group Montacuta is at once distinguishable from Loripes 

 by its much smaller size, and Lascza from Kellia by its reddish 

 colour. Our next genus had better be Diplodonta, of which our only 

 representative is a beautiful little shell, white and almost trans- 

 parent, equivalve and almost circular, with a double ligament more 

 or less external and with two teeth in each valve, the anterior in the 

 left and posterior in the right being bifid. Next to it we will place 

 Cyamium, the smallest of our bivalves, which can only be examined 

 under the microscope, and is recognisable at once by its ligament 

 rising conspicuously above the level of its dorsal line. There are 

 two other genera in which the ligament is obtrusively apparent, 

 though it is partially overlapped in a groove. These are Lucina, 

 which has the anterior scar narrow and projecting a long way within 

 the pallial line ; and Cyprina, in its solitary species, one of the 

 finest of British shells, thick, large, and heavy, in which the muscular 

 scars are so oval and shiny as to be characteristic, though it is 

 hardly worth while to look inside when the outside is so unmistakable. 



We have nearly finished with Division B. Only two genera 

 remain, and these used to be one. In them the ligament is incon- 

 spicuous, and the shell horny and translucent. One of them, 

 Spharium, has the beak almost midway ; the other, Pisidium, has 

 the beak near the anterior end. Both of them are freshwater. 

 They are the only British representatives of the Cyraeriidae. 



