240 MR. L. N. G. RAMSAY ON 
Here we have an interesting problem. JV. cyclurus and NV. fucata 
are admittedly closely allied in some respects. The former inhabits 
the North Pacific, the latter the North Atlantic. It is probable 
that at a former epoch the distribution of the two was continuous 
(around the north coast either of Eurasia or of America, or vid 
Panama). The existence of certain other closely allied and even 
identical species in the two regions points to this. Further, it 
seems probable that the hypothetical ancestor-species of the two 
had already formed its commensalistic habits before the distri- 
bution-area was severed (it would be a remarkable coincidence 
for the two allied species to have taken up the same habit 
independently—a habit not exhibited by any other nereid). 
Then, after the separation, the Pacific division of the ancestral 
Species, apparently under the same conditions (both inhabit 
Gastropod shells tenanted by species of Hupagurus, Pagurus, etc.), 
has made an enormous stride in evelution, the while that the 
Atlantic division has remained more or less stationary ! 
NEREIS VEXILLOSA Grube. 
Nereis vexillosa Grube, Ehlers (2), p. 573, Taf. xxiii. figs. 3-5. 
An example of this fine species, labelled “St. Paul Island, 
Alaska, 1897,” accompanies the collection. 
This form, originally described by Grube from the Sea of 
Okhotsk (3, p. 3, Taf. i. figs. 1, 5,6), is now known to inhabit also 
the west coast of America from Alaska to California. Izuka, in 
1912 (5, p. 173), described a very closely allied species from Japan, 
IV. ezoensis, He givesno reason for separating this, except that 
‘‘ the falcate end-piece of bristles [is] much longer in this species 
than in the closely allied Wereis vewillosa.” He figures a faleate 
bristle, but emits to state from what region of the body it is 
taken. Now, in JW. vewillesa, as in certain other nereids, the 
length of the end-piece of the falcate bristles decreases very 
markedly, proceeding from head to tail. In the specimen before 
the writer at present, the falcigers from the posterior portion of 
the body have certainly appendages very much shorter than that 
figured by Izuka, but in those from the anterior portion (e. g., 
10th parapod) they are nearly as elongate as in the figure. I can 
discern no other difference between the two species from Izuka’s 
description and his excellent figures, and would therefore suggest 
a more detailed comparison, with a view to determining the 
specific distinctions in a way more satisfactory to others. Apart 
from the progressive variation in length of the falcigers through- 
out the body of a single individual, that in different individuals 
must be taken into account. 
NEREIS PELAGICA L. 
Nereis pelagica L., Izuka (5), p. 154, pl. xvii. figs. 1-6. 
One specimen, dredged near Turn Island, Friday Harbour, 
Puget Sound, 18th July, 1911. 
