94 PHOLADID^. 



that on an average at least one-half is occupied by its 

 burrows. The Xylopliaga has never, like the Teredo^ been 

 observed by my correspondent to form a testaceous tube, or 

 lining to its cell. Many of the chambers of the Xylopliaga 

 before me are one inch and a half i)i length, thus exceed- 

 ing by one-half the longest noticed by Turton. The shells 

 of my largest specimens are 52 lines in length : the two 

 valves joined at the hinge occupy a space of h\ lines in di- 

 ameter. This species differs from Teredo navalis (Turton), 

 by boring against the grain of the wood (all of which is 

 pine), in a diagonal manner." 



Loven enumerates Xylophaga dorsalis among the inhabi- 

 tants of the seas of Norway. 



Only one more species of this genus {X. globosa) is as 

 yet known to us, which, judging from its very brief diag- 

 nosis in the Zoological Proceedings for 18o5, appears prin- 

 cipally to differ from our own by the greater slope of its 

 posterior dorsal margin. 



PHOLAS, LiNN^us. 



Valves oval or elongated, (rarely suborbicular,) equivalve, 

 inequilateral, gaping, especially anteriorly ; external sur- 

 faces of the shell more or less ornamented by transverse 

 laminated ridges and longitudinal furrows ; the inner sur- 

 faces presenting two distinct muscular impressions, very 

 distant, the posterior one largest, connected by a pallial 

 impression, which is deeply sinuated posteriorly, narrow, 

 oblique, and tongue-shaped centrally. A curved free 

 spoon-shaped apophysis springing from beneath the beaks 

 in each valve, and directed forwards. Beaks covered by 

 a callosity reflected from the interior. No hinge. Liga- 

 ment obscure or rudimentary. Accessory valves or plates 

 more or less deveIo])ed at the back of the shell. 



