BIGELOW: EXPLORATIONS OF THE COAST WATERS. 299 



southern edge of the continental shelf south of Marthas Vineyard 

 (Station 102G0). 



In 1915 swarms of Limacina halea were encountererl on three occa- 

 sions, first over the outer edge of the continental shelf south of Shel- 

 burne, Xova Scotia, June 23 (Station 10294); off Marthas Vineyard, 

 October 22 (Station 10333) and again in Massachusetts Bay, October 

 27 (Station 10338), while it occurred, in smaller numbers, at about 

 50% of the stations in the Gulf.' 



Although Limacina halea is a characteristic member of the Gulf 

 plankton, and though it has now been taken in all parts of the Gulf, 

 it is far less regular in its occurrence than Calanus, Euthemisto, or 

 Sagitta elegans. Thus it swarmed on German Bank in August 1912, 

 and in August 1913; but was not found there in August 1914, or in 

 either May, June, or September of 1915, though it occurred only a 

 few miles away (Stations 10270, 10310). At several localities, too, 

 it was present during one cruise in 1915, absent on the next, or vice 

 versa, for example, off Monhegan Island (Stations 10303, 10318); 

 off Matinicus (Stations 10276, 10287), and in the* northeast corner of 

 the Gulf (Stations 10273, 10287). And neither in 1912, 1914, nor 

 1915, was it as generally distributed in the Gulf as it was in 1913 

 (1915, p. 303, fig. 72). In spite of these anomalies, however, there is 

 some evidence that Limacina approaches the western and northern 

 coasts of the Gulf as the summer advances, for in 1915 none were 

 taken in the numerous hauls along the Maine coast until Septem- 

 ber; and it appeared in summer off Cape Ann (Station 10306) and 

 oft' Matinicus (Station 10287), where it was lacking in May (Stations 

 10266, 10276). 



We have usually found Limacina halea most abundant in hauls from 

 30-60 meters (1915, p. 305), absent on the surface. But it occurred 

 on the surface at four stations, twice in considerable numbers in 1914 

 (Stations 10247, 10264); at five in 1915 (Stations 10294, 10295, 10308, 

 10329, 10333), twice in swarms (Stations 10294, 10333). How- 

 ever, these surface catches were made at night, and it was never found 

 on the surface between sunrise and sunset. Apparently Limacina, like 

 other plankton organisms performs a more or less regular vertical mi- 

 gration, rising toward the surface at night, sinking again at daylight. 



The salinities at which Limacina was numerous in 1914 and 1915 

 range from about 31.009co (Station 10295) to at least 35%o (Stations 



' Limacina halea was taken at the following stations in 1915: 10269, 10270, 10272, 10273, 

 10278, 10279, 10287, 10288, 10291, 10294, 10295. 10300. 10307. 10308, 10309, 10310, 10317, 

 10318, 10320, 10321, 10.328, 10.329, 103.30, 10331, 10332, 10.333. 10.337. 103.38. 



