300 bulletin: museum of comparative zoology. 



10220 and 10261); the temperature from about 5°-6°; to about 

 16.6° (Station 10264, surface), i. e., it was scarce, or absent both in 

 the coldest and warmest, and in the very freshest and sal test, water; 

 which corroborates the previous records (1915, p. 305). Its rarity 

 in Gulf Stream water (Station 10218), while it was common at other 

 stations on the slope where hydrography and plankton were less 

 purely oceanic (Stations 10220, 10261), indicates that the junction of 

 coast and oceanic waters is its normal southern limit, at least in the 

 upper layers, which agrees with the experience in 1913 (1915, p. 305) 

 as well as with its distribution in other regions ; and its absence in the 

 southeastern part of the Gulf, in the Eastern Channel and in the ^ea 

 off Halifax is explicable on this basis, these regions being occupied 

 by unmistakable, though diluted tongues of offshore water (p. 196). 

 Similarly, its absence, or rarity, in the very cold water of the Cabot 

 Current, off Shelburne, certainly suggests that it does not reach the 

 Gulf by that route. 



But hydrography offers no apparent explanation for the irregularity 

 and sporadic nature of its occurrence within the Gulf. Hence, if its 

 distribution there be dependent on the physical environment, as is 

 no doubt the case in the last analysis, it probably is neither teinpera- 

 ture nor salinity alone which limits the existence of the adult, but a 

 complicated set of interacting phenomena. Possibly too high temper- 

 ature and salinity may directly prevent its reproduction: it may be 

 food supply which is the limiting factor; or warm or cold ocean cur- 

 rents may act as actual physical barriers to its dispersal. 



Limacina helicina being as good an index of Arctic water (Meisen- 

 heimer, 1905b; Murray and Hjort, 1912, p. 6^0) as L. balea is for 

 boreal, its occurrences in our waters are of great interest: It has never 

 been found in the Gulf of Maine in summer (1914a, 1915); nor 

 in the western half of the latter at any season. But on our May 

 cruise, in 1915, it occurred at the two stations on the eastern side 

 where salinity (p. 224), temperature (p. 215), and other Arctic organ- 

 isms (p. 248) gave unmistakable evidence of Cabot Current water 

 (Station 10270, 150-0, and 50-0 meters, two specimens; 10272, 60-0 

 meters, seven specimens). 



We did not find it again in the Gulf; but in June it occurred at two 

 stations off Shelburne, one close to the land (Station 10291, 85-0 

 meters, five specimens), the other over the continental slope (Station 

 10295, 500-0 meters, two specimens). In August, 1914, it occurred 

 twice near land, off Halifax (Station 10236, 65-0 meters, two speci- 

 mens; Station 10237, 75-0 meters, two specimens). 



The third characteristic pteropod of northern waters, Clione lima- 



