MULTIVALVES AND BIVALVES. 39 



Saxicava, in which it is broken up. One genus, Scrobicularia, has 

 the beaks turned away from the fore end ; another, which has a 

 wrinkled periostracum extending over the siphons, is in this group 

 only as regards one of its species, Mya binghami. One shell, Castro- 

 chana dubia, has a long, calcareous sheath ; one, Petricola, is long 

 and thin and prickly, and looks like a narrow Pholas ; one, Venerupis, 

 is decorated with concentric rows of plates almost as thin as paper ; 

 and the tenth, Psammobia, is striated concentrically, and also (more 

 or less faintly) radially, particularly at one end in the case of two of 

 the species, ferroensis and costulata, besides being distinguishable by 

 its flat, narrow shape, and its beautiful rays of subdued pinks and 

 purples. 



Nothing would be gained by here tabulating this rather mis- 

 cellaneous lot, and we can proceed to our last group in which the 

 shell does not gape at either end. These we can divide into those 

 having the ligament partly external and partly internal and those in 

 which it is wholly external. Thus : 



Ligament partly external 



Teeth unequal in both valves Mactra. 



Teeth unequal in right valve only A mphidesma. 



Concerning which it may be worth noting that one only Amphi- 

 desma is reddish brown, while all the species of Mactra are white or 

 pale stone colour. We are left with those shells in which the 

 ligament is entirely external, and these can be sorted out into : 



Teeth two in right valve 



Inside margins notched Donax. 

 Inside margins not notched 



Shell with a white ray Donax politus. 

 Teeth in left valve unequal Gastrana. 

 Teeth in left valve equal 



Shell as broad as long Lucinopsis. 

 Shell not so broad as long Tellina. 



Teeth three in both valves 



Sinus rounded Tapes. 

 Sinus angular Venus. 



Here the only difficulty is with Donax, two species of which have 

 toothed edges, while the other has the edges smooth but is at a 

 glance recognisable by the white ray extending from the beak to the 

 ventral margin. Another Donax, trunculus, can be picked out at 

 once by the curious manner in which it is cut off short, making it 

 look like half a shell, so that among the bivalves it is what the 

 sunfish is among fishes. Both Gastrana and Lucinopsis are thin and 

 fragile; and of the last pair Tapes can be distinguished from Venus 

 by its being more oval and oblique, its only species approaching to 

 a roundish form being decussatus, which can be identified among the 

 crowd by the bold and numerous radiating striae which seem to 

 ripple from its beaks. 



Hitherto we have been dealing with the shells ; let us now direct 

 our attention to those who live in them. Let us leave conchology, 



