182 



is elongated, and gradually increased in diameter, as the 

 animal recedes further into the wood." 



Mr. Sowerby refers to the recent species, T. navalis, 

 and to the fossil species which he terms T. antenautce, from 

 their having existed before ships were invented, he refers 

 all those found in calcareous or siliceous wood. Among the 

 former he has seen none which show any of the accessory 

 valves, except the tube and the two spatulate valves 

 attached to the animal. Even the membrane that is pre- 

 served in the fossil ones covering the proper shells, and 

 attaching them to the outside of the tube, is not discoverable 

 in the recent specimens.* 



Pholadairice. Bivalves, either with or without acces- 

 sory pieces, and gaping anteriorly. PL viii. fig. 25. 



1. Pholas. An equivalved, transverse bivalve, gaping 

 on each side ; with various shelly accessory pieces placed 

 above or beneath the hinge ; the posterior or inferior border 

 of the valves reflected upwards. Recent. 



2. Gastrochcena. An equivalved, subcuneiformed bi- 

 valve ; the opening anteriorly oblique, very large and oval ; 

 posteriorly, hardly any. The hinge linear, marginal, and 

 without teeth. 



This genus of Spengler differs from the preceding chiefly 

 in being without accessory pieces. 



SolenacecB. Bivalves elongated transversely, without 

 accessory pieces, and gaping only at the lateral extremities ; 

 the ligament external. 



1. Solen. An equivalved bivalve, elongated trans- 

 versely ; gaping at the ends ; the beaks very small, not pro- 



* Mineral Conchology, Vol. ii. p. 229. The accompanying 

 figures, in that work, PL cii. are very satisfactorily illustrative of 

 the nature of this genus. 



