CONTRIBUTIONS OF OTHER ANIMALS 247 



clam (^Mya arcnaria Linn.) is in northern regions a species 

 which adds rapidly to the growth of the mud banks so com- 

 monly lying between the margins of the marine marsh shelf 

 and the edge of the deeper tidal streams. This species 

 has the habit of abiding beneath the surface of the mud, 

 holding communication with the water by means of its long 

 protrudable siphon. Owing to its curious habit of life the 

 shells of this species are not visible on the mud flats, and 

 are not often washed ashore, and thus the important work 

 performed by this creature in shoaling the water, a work in 

 which it is only less important than the oyster, is apt to be 

 unnoticed by the casual observer. The Pectens or scallops 

 also play an important role in sedimentation, but the fields 

 on which they grow are much more limited thai; those of the 

 species above mentioned. Owing to the lightness of their 

 shells they are more commonly swept against the harbor 

 beaches than any other species, except some of the tropical 

 kinds. 



Besides mollusks there are numerous other animals which 

 make important contributions to the detritus accumulating 

 in our harbors. Of these the crustaceans are the most note- 

 worthy. The crabs, lobsters, and their lower kindred, the 

 shrimps and sand hoppers, grow rapidly, exist in great num- 

 bers, and at their death leave tolerably solid skeletons as 

 contributions to the growing sheet of sediment. Much of 

 the exceeding fertility which characterizes all marine marsh 

 lands won to tillage is due to the phosphate of lime which 

 exists in the shells of these creatures whose forms have been 

 buried in the deposit. A group of lowly organisms known 

 as Foraminifera affords a host of small and swift sfrowinsf 

 forms which secrete hard skeletons, mostly of limy matter. 



