AEQUORIDAE. 171 



aequorid. The specimens on which both authors based their diagnoses were 

 preserved and therefore undoubtedly more or less contracted ; furthermore, 

 neither Maas's figure of M. macrodactylum nor the figures given by 

 Browne (: 05 b , PI. 2, figs. 11-15) of M. pensile seem to me to indicate such 

 a condition ; indeed, in the former the gastric wall is but slightly nar- 

 rower than in the Atlantic species referred by Maas to Aequorea for- 

 skalea (: 04% PL 2, figs. 12-H). Finally, my own investigations on living 

 specimens of three species identified as A. macrodactylum, A. coerules- 

 cens, and A. tenuis, and on an undescribed species from the Gulf Stream, 

 show that the mouth may rapidly pass from the Aequorea to the Meso- 

 nema condition ; and, perhaps most important, that when in the latter 

 condition, whether normally or as the result of preservation, there is often 

 no evidence, either in plications, folds, or obvious thickening, of contrac- 

 tion. Thus the Mesonema condition might readily be taken for the usual 

 one even if only momentarily assumed. 



Though I am forced to conclude from these experiments that the 

 diagnosis of the two genera as expressed by Browne (: 05 b ) and by Maas 

 (: 05) is artificial, yet I believe it by no means improbable that the degree 

 of development of the mouth may yet be of generic importance in species, 

 such as A. (Zyc/oJaclyla) groenlandica, in which it is extensible as a long 

 manubrium or curtain. However, until some student is able to examine 

 larger series of living specimens I doubt whether any generic diagnosis 

 founded chiefly on this character will be of much value. 



To retain any of the previous generic distinctions not only preserves an 

 unnatural order, but results in making any sound specific identification 

 almost impossible, since different individuals of a single swarm, or even one 

 individual under different external conditions or in different stages of con- 

 traction, might readily be referred to different genera; but to attempt 

 any generic revision of the members of this family without studying a 

 much larger series of living specimens than that to which I have had 

 access seems futile. It is to be hoped that such a study will soon be 

 made. In the meantime I believe that the most satisfactory course is to 

 recognize only one genus, Aequorea ; though at the same time admitting 

 that this reduction is probably excessive, and that it is only temporary. 



In identifying the two species of Aequoridae in the present collection 

 I have necessarily attempted a provisional revision of the Indo-Pacific 

 species; and although this is by no means final, I give it here, with- 



