GEOGRAPHICAL DISTRIBUTION. 227 



of intermediate Medusae are practically universal in oceanic regions, yet there 

 is reason to believe that some few may be local, or even littoral. A case 

 in point is Ptychogena eri/tlirogonou. This species was taken only in a re- 

 stricted area off the coast of Peru, and although fairly common, never at 

 any great distance from the land. In all probability, judging from analogy 

 with its natural allies, the species passes through a hydroid stage; and if 

 this be the case it is probably the hydroid, not the medusa, which is the 

 factor limiting its dispersal. 



Apart from the undoubted holoplanktonic forms but few species of Hydro- 

 medusae were taken during the oceanic portion of the cruise, the only species 

 which can be classed here, under the supposition that they pass through a 

 fixed hydroid stage, being Tiara pa'pua, Aeqmrea macrodadylum, Aequorea 

 coerulescens, Eirene viridula, Purena hrownei, Gonionemus suvaensis, and Olindias 

 singularis. All of these were previously known from the Pacific or from the 

 Indian ocean, or from both ; none however, except Eirene viridula, are as 

 yet known certainly fi'om the Atlantic, although all are represented there by 

 close allies. 



The collection made in Acapulco Harbor is most instructive on account of 

 the affinities of its members ; for of the fifteen species previously known, 

 excluding the holoplanktonic genera Liriope, Aglaura, Solmundella, and 

 Cunoctantha, only seven were previously recorded from either the Pacific or 

 Indian oceans alone ; one, Prohosci/dactyla ornata, is known in its two varie- 

 ties from both Indian and Atlantic oceans ; while no less than seven species, 

 Edopleura ochracea, PhiaUdium discoida, Phialium, duodecimalis, Amphinema 

 turrida, Amphinema australis, Lymnorca alexandri, and (probably) Zandea gem- 

 niosa, are known from the Atlantic alone. Furthermore, .all of these Atlantic 

 forms, with the possible exception of Edopleura ochracea, are, so far as we yet 

 know, limited to the western coast of the Atlantic south of Cape Cod. And 

 the same is true also of the Atlantic distribution of Prohoscydadyla ornata; 

 while Stomotoca divisa, not taken in Acapulco Harbor itself but off the coast of 

 Mexico and in the Gulf of Panama, is so closely allied to the West Indian *S^. 

 pterophjlla as to be hardly separable from it (p. 202). Up to the present 

 time our knowledge of the leptoline Medusa fauna of the tropical west coast 

 of America, north or south, has been almost nil ; while the same is true 

 to-day of that of the east coast of Mexico, and of the north and northeast 

 coasts of South America. 



Mayer has already called attention to a resemblance between the lepto- 



