259 



Ps. strigilata were procured about Exmouth in a living state. Since 

 that time, not a single specimen has been found. Like the shoals 

 of migratory fish, it is 'probable that particular species of shell- 

 fish resort occasionally to particular localities, and as abruptly dis- 

 appear. Our cabinet contains some specimens of the Psammobia 

 taeniata, procured at Exmouth more than twenty years ago, at which 

 time it is supposed they were not uncommon, but since which time 

 not an individual has been found. 



Arca reticulata, p. 168. 



By the kindness of Dr. Mac Gee, of Belfast, this very beautiful 

 shell is now before us. The breadth is more than twice as much as 

 the length. One end is rounded, and the other obliquely truncate. 

 The epidermis is of a pale chesnut, darker towards the ends, under 

 which it is glossy white. From our Arca perforans it differs, in 

 being much flatter, thinner, and semitransparent, with the beaks 

 nearer the rounded end. Besides the fine decussations which cover 

 the surface, there are about twelve raised longitudinal lines clothed 

 with fine dark hair. The inside is striate longitudinally ; and what 

 specifically distinguishes it both from the Arca perforans and the 

 Arca barbata, the inner margin is finely notched. 



Dr. Mac Gee also informs us, that the Mya nitida, described by 

 Mr. Wood, p. 1 03, with an obtuse tooth in each valve, has been 

 found at Bangor, in Ireland. 



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