PHOLADIDEA. 115 



be an exotic species of Parapholas, perhaps the ovoideus 

 of Gould. That genus is distinguished by having two 

 furrows'. 



Genus II. PHOLADI'DEA*, Goodall. PL IV. f. 2. 



BODY oblong, rather thin, capable of being contained within 

 the shell : tubes united throughout and terminating in a disk, 

 enveloped in a fine membranous retractile sheath ; the orifice 

 of the larger tube is cirrous, that of the smaller one plain : 

 gills very unequal : palps long and narrow : foot, in the young 

 and half-grown state very large, truncated, and springing 

 from a long stalk in the centre of the body ; in the full-grown 

 state it becomes atrophied, and is reduced to a mere point. 



SHELL oval, semitransparent but lustreless ; anterior part 

 covered with prickly ridges ; in the adult the front gape is 

 closed by a shelly dome or convex plate, and the posterior end 

 is furnished with a cup-shaped appendage, which has a texture 

 between shell and membrane : beaks much inflected, and con- 

 cealed (but not covered) by a fold of the hinge-plate : teeth 

 conspicuous, triangular : apophyses long, and partly concealed 

 within the hinge: dorsal shields two, formed in the adult only ; 

 they are very small and triangular, placed close to the hinge on 

 the anterior side, and in a line with the fold of the hinge-plate. 



The distinctive characters of this genus are rather 

 physiological and conchological than malacological ; 

 they are not developed until the Pholadidea has attained 

 its full growth. In the young and immature state it 

 does not differ from Pholas. The same peculiarity is 

 found in Martesia, Jouannetia, and other allied genera. 

 Mr. Berkeley has suggested to me that the cup-shaped 

 appendage may be the homologue of the pallets in 

 Teredo. It certainly occupies the same place in the 

 animal ; and both serve to protect the entrance of the 

 hole, although less efficaciously in Pholadidea than in 



* Having the shape of a Pholas. 



