320 



BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



gin. The leaflike gonads, too, are unknown among Halicreids, and 

 the radial canals are much broader in that family than in Nauarchus. 

 The resemblance between the two in the flat stomach and simple 

 lip, is certainly far less significant, phylogenetically, than the struc- 

 ture of the marginal organs ; in short, it is merely superficial. The 

 only Trachomedusae with which it shares its leaflike gonads are the 

 Geryonids, but here again we find essential differences; that is, ab- 

 sense of peduncle, free sense clubs, structure of tentacles. And the 

 structure of tentacles and gonads separate it from the Trachynemidae. 



The location of Nauarchus in the Petasidae is based chiefly on the 

 structure of the tentacles, which agree very closely with those of 

 Eperetmus (Bigelow 19155), both anatomically and in their relation 

 to the bell margin. And, except for the replacement of sucking disk 

 by nematocyst knob and in the details of the nematocyst rings, they 

 agree with those of Gonionemus and the primary tentacles of Olin- 

 dias. 



The otocysts, too, are easily reducible to the ordinary Olindiid type. 



According to the subdivision of the family proposed by Browne 

 (1904) and followed here (p. 317, 1909a, 1912, 19156), Nauarchus be- 

 longs to the Petasinae. [For a tabular view of the Olindiinae, see 

 Bigelow, 1915&, p. 400.] But its leaflike gonads and the presence of 

 six radial canals separate it from the only other Medusae which fall 

 into the Petasinae as here defined, namely, Petasus, Dipetasus, Peta- 

 sata, and Petachnum of Haeckel. All these are united by Mayer 

 (1910) as Petasus, but until specimens agreeing with Haeckel 's ac- 

 count are again discovered discussion of them is idle. 



NAUARCHUS HALIUS Bigelow. 

 Plate 43, figs. 1-5. 



Nauarchus halius Bigelow, 1912, p. 258. 



Nauarchus halius — material examined. 



All three were so badly crumpled that it was impossible to make a 

 photograph of the general habitus. But all were well preserved ana- 

 tomically. 



The species having been described elsewhere (1912) in detail, the 

 present account is limited to the features most important for their 

 bearing on the relationships of this remarkable genus. 



