PLATE CXXIV. 



notices the affinity 7'iisticum bears to the other species ; but observes 

 that the grooves are deeper and the ribs fewer, and more convex in 

 rusticum : he remarks also that the latter has a ridge on the anterior 

 margin when the valves are closed, and a narrow depressure behind 

 the beaks, " ano evidente, sed angusto," which is not in the other. 

 The grooves in our Shell are not so deep as " sulcis profundioribus'* 

 implies, but they are both deeper and wider than in the common 

 sort ; the ribs are rather more convex also, fewer in number, and 

 rugged, as Gmelin describes it. — ^The colour exactly corresponds : 

 he says it is sometimes ferruginous, with livid bands and sometimes 

 white, with the anterior part fuscous : another variety of it is white, 

 fasciated with a ferruginous yellowish or blueish colour. 



