12 ERNEST WARREN. 



(3) The presence of a small ectoderm-chamber above the 

 mouth : morphologically, this is almost of the nature of a 

 stomodffium. 



(4) The tentacles terminate in small, but pei'fectly distinct, 

 capitula, with well-developed nematocysts. 



These differences are undoubtedly considerable, but unfor- 

 tunately there is no general criterion for determining the 

 amount of difference allowable between the species of a genus, 

 and it has been thought unnecessary at the present time to 

 separate the species rigid a from the genus Bimeria. 



The genera Wriglitia, Parawrightia, Ciarveia, and 

 Bimeria are undoubtedly related to one another. In all 

 these genera there is a formation of a definite perisarc-cup 

 around the hydranth, and the gonophore is in tlie form of a 

 sporosac surrounded by a firm layer of perisarc. 



Since the above was written Dr. James Ritchie, of the 

 Royal Scottish Museum, has very kindly sent to me notes on 

 certain allied hydroids which are described in journa s 

 unobtainable in this country. He writes : " The species 

 which seems to me to come nearest to yours in habit 

 is Grarveia groenlandica Levlnsen. It bears branched 

 creeping hydrorhiza, from which simple polyps project. . . . 

 Bimeria corynopsis Vanhoffen, from Gauss Station, in the 

 Antarctic, has 4-5 polyps on a stem, and your specimens bear 

 no resemblance to this species." 



By placing the present species in the genus Bimeria, 

 Dr. Ritchie points out that no weight is given to the character 

 of the capitation of the tentacles, and, judging from a rough 

 sketch sent to him, he suggests that possibly the present 

 species may really fall into Pictet's genus Sphgerocoryne. 

 The habit of growth of Sph^rocoryne bedoti Pictet, from 

 Ambouia, is very similar to that of B. rigida, the tentacles 

 are capitate, and the shape of the hydranth is also much 

 the same. S. bedoti differs, however, in (1) the perisarc 

 stopping at the base of the hydranth, (2) the tentacles being 

 arranged in several compressed verticils, and (8) it is 

 believed that free-swimming medusa3 are formed, and these 



