522 REPORT OF COMMISSIONER OF FISH AND FISHERIES. [10] 
3. Zygodactyla. Tentacles more numerous than the radial tubes. 
It is evident from what we know of the development of the Medusa 
(gonophore) of Z. granlandica (2) that the relative number of tentacles 
and radial tubes varies with age, and consequently the three subgenera 
are difficult to separate on this feature alone. There are specimens of 
Polycanna in the collection with characters of the first subgenus Rha- 
costoma, to which I have already given the name P. americana. It is 
believed that we have at least two species of Polycanna on our New 
England coast, and provisionally these may be known as P. grenlandica 
and P. americana. The basis of the separation of the two is the exist- 
ence in the former of rows of subumbral knobs between the chymiferous 
tubes and the absence of these knobs in the latter. It happens that in 
the latter the number of tentacles is less than the number of chymif- 
erous tubes, while in the former, according to A. Agassiz, the number 
of tentacles is greater than that of the radial tubes.* 
Tt seems to me that the presence or absence of the subumbral knobs 
is a much safer character to rely upon’in the separation of our species 
of Polycanna than any which has yet been suggested. If new inves- 
tigation shall show that true specimens of grenlandica do not have 
subumbral knobs, our New England species is possibly new. From the 
fact that a supposed type specimen of Polycanna, labeled Z. grenlandica, 
in the collection of the Museum of Comparative Zoology, has these tu- 
bercles, the name grenlandica is retained for this species. 
There is another Zygodactyla-like Medusa in which I have not been 
able to find these gelatinous knobs, either in a live animal or in alcoholic 
representatives. As this species also differs from the species crassa and 
cyanea in the relative number of tentacles and chymiferous tubes, it is 
supposed to be the new species, americana. 
Unlike all other American Zygodactyle, as described by A. Agassiz, 
this species has a smaller number of tentacles than of radial tubes, and 
at the same time none of the alcoholic specimens have subumbral tuber- 
cles. It is possible that the former feature indicates an immature 
Medusa, but not so the latter; for, as has been already shown, the sub- 
umbral tubercles are present in the Medusa when very small. 
Specimens referred to P. americana were collected in the following 
localities : 
Pelalogne Station. | North latitude. | West longitude. 

o ‘ “ ° ’ “ 
11650 2563 39 18 30 71 23 30 
11665 2567 37 45 00 66 56 00 
11673 2566 37 23 00 68 08 00 
11674 2563 ? 39.18 30 71 23 30 
11677 2566 37 23 «(00 68 08 00 
211649 2539 39 59 45 70 53 00 


* The existence of radial subumbral knobs and a larger number of tentacles than 
radial tubes is supposed to characterize grenlandica, although the knobs are not 
mentioned in A, Agassiz’s description. 
