HYDRO MEDUSAE, SIPHONOPHORES, AND CTENOPHORES. 309 



is one of the longest known and most widely distributed geographi- 

 cally of medusan groups. The difficulty facing us is the extreme 

 variability of its members — the inconstancy of almost every char- 

 acter which might be expected to serve as the basis for classification. 



I have already argued, from a study of living as well as preserved 

 material (1909a, p. 171), that the size and structure of the mouth 

 as used by Haeckel (1879) and recently by Maas (1909) and Browne 

 (1904, 1905) is misleading as a generic character. Torrey (1909, pp. 

 28, 29) has simultaneously come to the same decision from his study 

 of living examples. Our view has been accepted by Mayer (1910) 

 and Vanhoffen (1911). Neppi and Stiasny (1913) have recently 

 corroborated it on large series of living specimens. But while I 

 recognized only one genus, Aequorea, to include the whole family, 

 exclusive of Haeckel's problematical genera Zygocanna and Zygo- 

 cannula, Mayer retains Stomobrachium for species with 12 canals 

 and Zygodactyly for forms with subumbral gelatinous papillae. 



I have recently studied excellent series of the latter (1915a), find- 

 ing the papillae as Mayer describes them. But in a group where it is 

 so difficult to separate even species it is better to use their occur- 

 rence as a specific, not a generic, character. And, at any rate, if the 

 papillate forms be recognized as a separate genus, the international 

 code of nomenclature forbids the use of the name Zygodactyla for 

 them, because its type species, Z. coerulescens Brandt, does not have 

 papillae (Bigelow, 1909a) ; hence, as Mayer himself points out, it 

 is an Aequorea. 



Stomobrachium may as well be left out of this discussion, for 

 neither the early descriptions (Brandt, 1838 ; A. Agassiz, 1865) nor the 

 recent account by Le Danois (1913) tells anything about its otocysts 

 or even whether it has any. 



Vanhoffen (1911) distinguishes two genera of " Vielstrahligen 

 Aequoriden," Aequorea and Mesonema, separating them solely on the 

 proportional number of tentacles and radial canals. But this diag- 

 nosis is unsatisfactory, because it leaves no place for species (or 

 specimens) with slightly fewer tentacles than canals, whereas such 

 proportional numbers have often been recorded. In short, I believe 

 that in the present stage of our knowledge all aequorids with the 

 canals normally simple (unbranched) must be grouped in one genus, 

 Aequorea, as distinguished from the forms with branched canals, 

 Zygocanna. 



The present collection contains an excellently preserved series of 

 the latter, one of its most interesting finds. 



As I have pointed out (1913, p. 36), the separation of distinct 

 species in the genus Aequorea is very difficult, except for groen- 

 landica, characterized by subumbral papillae; and tenuis (-\-flori- 



