218 THE MEDUSAE. 



and that the latter are considerably the larger. The number of tentacles, 

 when differing so little, as pointed out above, cannot be made the basis of 

 specific separation on the scanty evidence afforded by only four specimens ; 

 and size, except when proved to be constant in large series, is even more 

 unreliable, being a character subject to great variation in Medusae. As 

 an example of its variability I need only refer to the well-known case of 

 Ci/anea arctica. 



The present captures, from between 300 and 400 fathoms and the sur- 

 face, and that of the "Siboga" specimen from between about 475 fathoms 

 and the surface, show that Heterotiara is an intermediate form. Its hori- 

 zontal range is extended by the " Albatross " from the Malay Archipelago 

 to the Humboldt Current. 



Williidae Forbes, 1848. 

 sens. em. Browne ('96, : 04). 

 Hydrolaridae Allman, 1872. 



Anthomedusae with either four or six branched radial canals; tentacles 

 solid ; manubrium highly developed, separable into basal, gastric, and oral 

 portions, as among Leptomedusae ; with exumbral nettle strands. 



We owe to Browne ('96) the knowledge of the hydroid of Willia 

 steUata, and also the demonstration, from a study of sections, that the 

 gonads belong to the walls of the manubrium, not to the radial canals, and 

 that the family therefore belongs to the Anthomedusae, not to the Lepto- 

 medusae as Haeckel ('79) supposed. According to both Browne (: 04) and 

 Maas (: 05) two genera are to be distinguished in this family: — Willia with 

 six, Proboscidactyla with four, main radial canals. The present collection 

 contains examples of the latter only. 



Proboscidactyla Brandt, 1838. 

 sens. em. Browne, :04; Maas, :05. 



Williidae with four main radial canals. 



The specimens of this genua in the collection appear to represent the 



sexual stage of the form described by Maas (: 05) as P. favicirrata, var. 



■ i, which is no doubt the P. species of Huxley, for which Browne 



(: 04) lias proposed the name P. tropica. I cannot agree with Maas (: 05) 



thai this form is a variety of /'. flavidrrata Brandt, because the type of 



