Chap. IV. GENERA OF PELAGID.E. 127 



ocular and the tentacular pouches. The total number of lobes is forty-eight, thirty- 

 two of which are large, and sixteen small. In that respect, this genus resembles 

 Dactylometra, but it differs from it in having only three tentacles to each tentacular 

 lobe, which, considering the homologies of the structure of these segments of the 

 body, do not correspond to the three large tentacles of Dactylometra, but to the 

 middle large tentacles and the two small ones, combined with a great development 

 of the two middle lobes, while the lateral ones are almost rudimentary. From 

 the figure of Mertens', it would appear that the small marginal lobes belong to 

 the ocular, and not to the tentacular pouches. If this is trulj^ the case, this 

 constitutes an additional reason for separating generically Chrysaora melanaster from 

 Chrysaora helvola, as in that case the marginal structure of the radiating pouches 

 would be reversed in the two genera; the tentacular pouches branching into four 

 sacs in Polybostrycha, while there are only two in the ocular pouches; and four 

 sacs in the ocular pouches of Melanaster, two of which are large and two small, 

 and only two in the tentacular pouches. The two last genera are thus far only 

 known from the Pacific Ocean, Polybastrycha helvola between Sitka and the Aleu- 

 tian Islands, and Melanaster Mertensii on the coast of Kamtschatka. My son has 

 observed another species of each of these Uvo genera on the coast of California. 



The genus Dodecabostrycha of Brandt is passed over in this enumeration, as 

 it does not belong to the family of Pelagidte. The genus Heccaedecabostrycha 

 I have no means of characterizing. 



Zygonema Aff. Among the drawings made by Mr. J. Drayton, during the United 

 States Exploring Expedition under the command of Captain Charles Wilkes, I find 

 a Medusa, from the harbor of Rio Janeiro, represented under the name of Pelagia 

 volutata Couth., which evidently belongs to this family, but presents a combination 

 of characters not observed in the species thus far mentioned. All the segments 

 between the eyes show four larger lobes, subdivided by shallow indentation.s, from 

 which arise four tentacles. Such a combination of characters is only intelligible 

 on the supposition that, as in Pelagia jjroper, the marginal sacs of the ocular 

 pouches unite with the marginal sacs of the adjoining tentacular lobes, and that 

 each of the tentacular pouches has six marginal sacs, two of which are imited 

 with the sacs of the adjoining ocular jiouches, while two and two others, united 

 together, form two independent lobes. But vuitil this Medusa has been examined 

 anew, with reference to this point, the genus to which it belongs must remain 

 doubtful. 



