PLANKTON OF THE GULF OF MAINE 
273 
P. robusta, two stations outside the continental edge between the latitudes of 
Delaware Bay and New York, July, 1913 (stations 10064 and 10071); one station 
outside the edge off Shelburne, Nova Scotia, July 28, 1914 (station 10233); one 
Canadian fisheries expedition station outside the continental edge and three over 
the outer part of the shelf off Nova Scotia, July, 1915 (Willey, 1919) ; and one Michael 
Sars station east of the Grand Banks (Murray and Hjort, 1912, p. 654). 
P. xiphias, one station outside the continental edge off Delaware Bay, July 20, 
1913 (station 10071). The Canadian fisheries expedition of 1915 had it at one 
June station in deep water off the mouth of the Laurentian channel, one July station 
near Sambro Bank and one outside the continental edge off Cape Sable (Willey, 1919) ; 
it was also listed by Sars from the same Michael Sars station east of the Grand 
Banks which yielded gracilis and robusta (Murray and Hjort, 1912, p. 654). 
It is probable that when the ranges of these four Pleuromammas are better 
understood it will be found that all of them are universal away from land over the 
temperate and tropic latitudes of all oceans. Off the eastern coast of America, the 
continental edge and the outer part of the continental shelf would seem their normal 
inshore boundary, along which all of them may be expected in the warm, highly 
saline waters of the inner edge of the so-called “Gulf Stream” as far north as the 
Grand Banks; but the presence of abdominalis in the Gulf of St. Lawrence and the 
Gulf of Maine records to be mentioned next show that on occasion they may drift 
into distinctly neritic situations. 
One other species of the genus, P. boreale, is to be expected in the Gulf of Maine, 
having been found by the Canadian Fisheries Expedition of 1915 at five stations off 
Nova Scotia (Willey, 1919) side by side with the others; but as yet it has not been 
detected in the Gulf of Maine towings. 
The several Pleuromammas, like other planktonic animals which are purely 
immigrants, and uncommon ones, in the Gulf, have most often been found in the 
eastern side — that is, nearest their path of entrance (fig. 82) — and in the southwest 
part, which they may fairly be assumed to have reached via the anticlockwise eddy 
which dominates the circulation of the gulf. 
If the data so far obtained are fairly representative, abdominalis (only one 
record) is the least common of the four species in the Gulf of Maine, whereas it is 
the only Pleuromamma yet reported from the Gulf of St. Lawrence and the most 
common at San Diego (Esterly, 1905). Pleuromamma has been represented by 
scattering specimens in the Gulf of Maine tows, its numbers per square meter working 
out as follows for the spring stations of 1920: 
Species and station 
Number 
per square 
meter 
Species and station 
Number 
per square 
meter 
P. gracilis: 
P. xiphias: 
20056 
10 
20048 
10 
20103 
26 
20072 
50 
20114 
200 
20102 
9 
P. robusta: 
20117 
175 
20089 
50 
20098 
12 
