16 THE HYDROIDS. 
Versluys’s statement is, “ Ainsi, sur une coupe transversale d’un trone prin- 
cipal assez prés de son sommet, je compte trente ou trente-et-un tubes 
périphériques, entourant le tube axial d’une couche triple.” The size of the 
pinnules and the number and arrangement of their peripheral tubules is like 
that given by Versluys. 
In the short uncovered, distal portions of the axial tube the hydrothecae 
are attached to the stem in their basal part for nearly half their entire 
length. After a brief soaking in caustic potash they are partly separated 
(Pl. 11, fig. 3), while after boiling in caustic potash they become entirely 
free (Pl. 11, fig. 4; Pl. 12). Pictet and Bedot have already called attention 
to this fact. It is also to be noted that after this treatment the hydro- 
thecae of the main stem appear somewhat pedunculated, while those of the — 
pinnae do not (PI. 12; Pl. 13, fig. 7). 
The two localities for this species are off the Galapagos Islands in the 
Pacific, and off the Testigos Islands some fifty miles north of Venezuela in 
the Caribbean Sea. This is another of those interesting cases where the 
same marine form is found on both the Atlantic and Pacific sides of the 
Isthmus of Panama to which Mr. Alexander Agassiz first called attention in 
1888. 
Kirkpatrick in 1890 wrote, “It would appear, then, that Perisiphonia is 
synonymous with Zygophylax.’”’ I agree with him, and with Jiiderholm, 
that the slight difference of the continuation of the peripheral tubes quite or 
not quite to the distal ends of the stem and branches is not a sufficient basis — 
for’ generic distinction, and therefore the species described as Perisiphonia 
must be classified as of the genus Zygophylax. This genus now comprises 
these seven species : — 
Zygophylax profunda Quelch, 1885, Cape Verde Islands. . . . . . . . 4500 fathoms 
Zygophylax (Perisiphonia) pectinata Allman, 1888, New Zealand. . . . . 700 
Zygophylax (Perisiphonia) pectinata Pictet and Bedot 1900, Gulf of Gascogny 300 metres 
Zygophylax-(Perisiphonia) pectinata Pictet and Bedot, 1900, Azores . . 818: 78 
Zygophylax pectinata, Jaderholm, 1903, Eastern Atlantic (about 500 ntl 
west of Gibraltar) . . . . . 162 fathoms 
Zygophylax operculata Ji idenibiag 1903, Petaronn! Cape Valeiens ~ + eo eta 
Zygophylax (Perisiphonia) filicula Allman, 1888, Azores. . . . .. . . 400 & 
Zygophylax (Perisiphonia) filicula Allman, 1888, Australia... . . . 150 & 
Zygophylax tizardensis Kirkpatrick, 1890, China Sea. . . 1. . .... So & 
Zygophylax chazaliei Versluys, 1899, Testigos Island . .. .. . =. . 80 metres 
Zygophylax chazaliei Clarke, 1907, Hood Island . .... . . . . . 800 fathoms 
Zygophylax biarmata Billard, 1905, Bay of Biseay. . . . . .. . . . not given 
