HYDROMEDUSAE, SIPHONOPHORES, AND CTENOPHORES. 323 



a fact that led to the suggestion that they might be lacking in this 

 genus (Bigelow, 1909a). But I have found one, in one of the present 

 examples, a particularly fragmentary example, as it happens. The 

 sense organ is a free club, much like that of Aglantha, standing free 

 on the margin, midway between two tentacles. Its structure is not 

 remarkable, there being an entodermal core with a terminal mass of 

 concretions. It is not in very good condition. It stands on a slight 

 elevation of the margin, and though I could find no other otocysts 

 in any of the specimens, several of them showed the elevations, alter- 

 nating with the tentacles. 



The present series supports the view that the length of the 

 manubrium is so variable as to be quite worthless as a specific cri- 

 terion. The extremes illustrated here are a specimen 26 mm. high, 

 in which it is only 1 mm. long; and two of 27 mm. and 29 mm., 

 respectively, in which it hangs to the bell opening ; and as there is a 

 series of intermediates between these two, the differences are merely 

 evidences of contraction and expansion. In specimens not too much 

 contracted, and in good condition, the mouth is surrounded by four 

 prominent lips. But under extreme contraction, or in very limp 

 specimens, these may be masked. 



I examined all the specimens without finding any variation, in any 

 octant, from the normal number of tentacles, i. e. two adradials and 

 one interradial. In the smaller examples the latter is much the 

 smallest, but in the larger ones all are of nearly the same size. 



Genus RHOPALONEMA Gegenbaur, 1856. Sens. em. Vanhoffen, 1902. 



The studies of Maas (1893) and especially of Vanhoffen (1902) 

 have given us an excellent concept of the structure and limits of 

 Rhopalonema. I have previously (1909a) discussed this genus at 

 some length, and given my reasons for concluding that all species 

 referable to Rhopalonema which have yet been described belong 

 either to velatum or to the species from the mid depths variously 

 known as funerarium or coeruleum — a view which I share with 

 Vanhoffen (1902) and with Maas (1905). 



RHOPALONEMA VELATUM Gegenbaur. 



Rhopalonema velatum Gegenbaur, 1856, p. 251, pi. 9, figs. 1-5. 

 Bigelow 1909a, p. 129, and Mayer 1910, p. 378. 



Rhopalonema velatum — material examined. 



Synonymy, 



74841°— 19— Bull. 100, pt. 5- 



