PLANKTON OF THE GULF OF MAINE 447 



Rhizosolenia shrubsolei was sparsely represented off Cape Cod and near Mount 

 Desert Island early in October, 1915 (stations 10323 and 10328), and on the north- 

 east and southeast parts of Georges Bank in July, 1914 (stations 10220 and 10224) ; 

 it swarmed on the western end of the bank on July 23, 1916 (station 10348), and 

 likewise in Nantucket Sound on October 25, 1915 (station 10335). Fritz (1921) 

 lists it regularly from St. Andrews through October and November, occasionally 

 in December, and not at all during the other months of the year, but Fish (1925) 

 found it flowering in midsummer at Woods Hole, as well as in winter. Rh. imbricata, 

 if it be actually separable from shrubsolei, which Gran (1908) doubts, was detected 

 by Doctor Mann at one station on the western part of Georges Bank on February 

 22, 1920 (station 2004(1). 



We have found Rhizosolenias of the alata-obtusa group (critical examination of 

 them is needed before they can be referred definitely to one or the other species or 

 variety) in small numbers on and south of Georges Bank in July (stations 10215 and 

 10220 in 1914, and 10348 in 1916), and once in abundance in the deep water a few 

 miles to the north of the bank during that month (station 10058, July 8, 1913). 

 There are no other summer records for them in the basin of the gulf, but they 

 dominated the moderately abundant diatom plankton at most of the stations occupied 

 by the Halcyon in the outer part of Massachusetts Bay from August 22 to 24 in 

 1922 (stations 10631 to 10642), though not in Cape Cod Bay (stations 10643 and 

 10645); likewise off Mount Desert Island on July 19, 1915 (station 10302). Fritz 

 (1921) noted them (occasional cells) at St. Andrews on August 28, regularly during 

 the last half of September, October, and November, but not in any other month. 

 We have no autumnal record for the alata-obtusa group in the open gulf but they 

 were detected at three stations (10493, 10496, and 10497) along the coast between 

 Cape Ann and Mount Desert from December 30, 1920, to January 1, 1921. In 

 1925 they were flowering in great abundance in the eastern side of Cape Cod Bay 

 and in the channel between Cape Cod and Stellwagen Bank from December 16 to 

 February 6 to 7 (Fish Hawk stations 2, 4, 6, and 7, trips 3, 5, 6, and 7, p. 396), after 

 which date they were only occasional, being succeeded by Thalassiosira (p. 396). We 

 also have record of them in the North Channel (station 20105), in the Eastern Chan- 

 nel (station 20107), and in the center of the gulf (station 20113) in April, 1920. 



This completes the list of Rhizosolenias so far recognized in the towings from 

 the outer waters of the Gulf of Maine. Fritz (1921) , however, also lists Rh. faroensis 

 occasionally in August and October at St. Andrews. In general, the genus Rhizo- 

 solenia is far less important a factor in the phytoplankton of the offshore waters of 

 the Gulf of Maine than in the open North Atlantic, where, as Cleve (1900) long ago 

 pointed out and as Ostenfeld (1913, p. 444) has recently remarked afresh, this genus 

 may be its most abundant member, a difference to be expected because most of the 

 species of Rhizosolenia, and especially Rh. styliformis (p. 444), are oceanic in nature. 

 As noted above (p. 396), however, rich flowerings of the genus (Rh. alata) in the inner 

 parts of Massachusetts Bay during the winter of 1924-25 suggest greater importance 

 for its neritic members close to the coast. 

 75898—26 29 



