DACTYLINA. 325 



Genus PHOLAS, Linnaeus. 



Siphons simple at the hase. 



Shell elongated, cylindrical, with the anterior gape always 

 open; dorsal valves two, the anterior single, central, and 

 lanceolate, the posterior small and transverse; the hinge- 

 plate reflexed over the heaks closely applied; pallial sinus 

 long and deep. 



Ex. P. costata, Linnmus, pi. 89, fig. 1, 1, a. 



In the genus Pholas, as restricted, there are two dorsal 

 plates, one anterior and one posterior, whereas in Dactylina 

 the accessory pieces are placed side hy side. 



Siwcies of Pholas. 



costata, Linn. patula, Gould. 



crucifera, Sow. truncata, Say. 



latissima, Sow. 



Genus DACTYLINA, Gray. 



Siphons naked at the base, the orifice of the branchial 

 cirrhated, the anal simple, or crenate at the margin. 



Shell oblong- ovate, valves with the front gape always 

 open ; dorsal valves two, lanceolate, placed side by side; tbe 

 hinge-plate reflexed over the beaks cellular beneath. 



Syn. Thovana, Leach. 



Ex. D. dactylus, Linnaus, pi. 89, fig. 2. Shell, D. dac- 

 tylus, fig. 2, a, 2, b. 



In this genus there is a small transverse plate behind the 

 two dorsal accessory valves, and the space behind, between 



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