70 GUIDE TO LOCALITIES. 



up by storm waves. The shell is recognizable by the general 

 smoothness of its surface, and by the peculiar crescentic groove 

 which leads downward from the umbilicus, parallel to the inner lip. 



One of the best methods of collecting these small species, as well 

 as the small pelecypods associated with them, is to skim off the 

 surface layer of sand and shells, to be sorted at leisure. In this 

 way much valuable time will be saved. 



Littorina littorea is common on the more stony portion of the 

 beach south of the Bath house, but is found frequently on the 

 sandy part of the beach. This species, as is well known, is not 

 a native ; but has been introduced from the west European coast, 

 coming to Massachusetts probably by way of Halifax, N. S. 



Of pulmonate gastropods, the little Melampus bidentatus is the 

 only one commonly associated with the molluscs on the beach. 

 The shells of this species are more abundant near high- water mark 

 than on the strand. The animal always maybe found in the marsh 

 lands behind the beach, where the tide occasionally overflows. In 

 the marshy lands about Oak island, near the centre of the beach, 

 it is a common form. It may be recognized by its resemblance to 

 Oliva, by the two folds on the inner enameled lip, and by its thin 

 translucent character and brownish horn color. 



Of the polyplacophora, Chiton (Trachydermon) ruber is the only 

 one likely to be found. This must be looked for among the roots 

 of the cast-up Laminaria. 



Pelecypods occur more numerously and in greater variety. Along 

 the upper part of the beach Cyprina islandica almost always may 

 be found, brought up from deep water during storms. When fresh 

 the shells are covered by chestnut-brown epidermis, but after a 

 period of exposure this is worn off. In addition to the epidermis, 

 f,he subcircular outline of the valves, strongly forward-pointing 

 beaks, dark, nearly black, external ligament, and absence of pal- 

 lial sinus are distinctive marks. Mactra solidissima is another 

 large pelecypod, common along the upper part of the beach. The 

 animal lives in the sand below low water and can be dug at very 

 low tide. This is the giant, beach, or dipper clam of the fisher- 

 men, and is esteemed as an article of food. Its large size, trans- 

 versely ovate, somewhat triangular form, and peculiar spoon-shaped 

 ligamental area separate it from all shells except M. oralis, 

 which is frequently, though more rarely, found in similar situa- 

 tions. This species may be distinguished by its smaller size, 



