THE MUSSEL OR CLAM AND OTHER BIVALVES 157 



namented with radiating ribs. Near the umbo are two projec- 

 tions, the " ears," characteristic of all the shells of the genus 

 Pecten to which the scallops belong. 



The outline of Pecten has been considerably employed in con- 

 ventional designs for mural decorations; indeed, the figure of a 

 well-known Mediterranean pecten (P. jacobins), found com- 

 monly in Palestine, became an emblem of religious significance 

 during the middle ages. Returning crusaders fastened to their 

 garments a specimen of " St. James's shell " as an evidence of the 

 fact that they had been to the Holy Land, and the design of the 

 shell came to be adopted upon many coats of arms and also in 

 the insignia of various orders of devout and adventurous knights 

 of the middle ages (Arnold). 



Classification of Mussels and Clams. — The mussels, clams, 

 scallops, and similar bivalves belong to the class Pelecypoda of 

 the phylum Mollusca. They possess a shell consisting of two 

 valves, a bilobed mantle, and leaf-like gills. There is no head 

 and no rasping organ (radula, see p. 161) in the mouth. They are 

 all aquatic and mostly marine. The four orders into which the 

 class is divided are separated largely on the characteristics of the 

 gills. They are very similar and so need not be given here. 



REFERENCES 



The Cambridge Natural History, Vol. III. — The Macmillan Co., N. Y. 



City. 

 Bulletins of the U. S. Fish Commission. 

 The Sea Beach at Ebb Tide, by A. F. Arnold. — The Century Co., N. Y. 



City. 



