REPORT ON THE SIPHONOPHORjE. 61 



Eegarding the phylogenetic origin of Porpema, we may simply assume that it has 

 been derived from the ancestral genus Porpalia, by multiplication and equalisation of 

 the submarginal tentacles, which form circular girdles, without octoradial arrangement. 



Porpema medusa, n. sp. (PI. XLVIL). 



Habitat.— South Atlantic, Station 327; March 4, 1876; lat, 36° 48' S., long. 

 42° 45' W. Surface. 



Umbrella (fig. 1, from above; figs. 2 and 3, in profile, fig. 2 with tentacles, fig. 3 

 after removal of them ; fig. 4, meridional section). — The umbrella is nearly spherical, and 

 has a diameter of 4 to 6 mm. (without the tentacles and the siphon). In some specimens 

 the vertical main axis is somewhat shorter than the equatorial diameter, in other 

 specimens a little longer. 



Exumbrella (figs. 1, 2, 3, ue). — The superior or apical face is slightly convex, cap- 

 shaped, with a flat annular furrow which separates the central disc from the elevated 

 margin, like a flat hat with a recurved brim. The central disc exhibits a dark pigment- 

 star with sixteen broad rays, and a greater number of finer rays between the dark 

 prominent main rays. A great number of stigmata is arranged in radial rows. 



Limbus Umbrellas (figs. 1-4, uu). — The free prominent border or margin of the 

 umbrella is nearly horizontally expanded, and slightly reflected upwards, like the broad 

 brim of a flat hat ; it is concave above, convex below. The breadth of the margin equals 

 the diameter of the exumbrellar central disc, and half the ecmatorial diameter of the 

 subglobular umbrella. The thickened edge of the margin is beset with a series of the 

 usual muciparous glands (compare above). 



Subumbrella (figs. 1-4, w). — The inferior or basal face of the umbrella exhibits 

 beyond the deep submarginal ring-furrow a narrow, smooth zone, which is radially 

 striped ; these stripes are the radial canals ascending from the corona of tentacles to the 

 margin of the umbrella. The broad tentacular zone of the subumbrella (fig. 3) is a 

 circular girdle, which embraces the equatorial zone of the whole umbrella ; its vertical 

 height equals the radius of the latter. After removal of the tentacles it appears elegantly 

 panelled (fig. 3, tu). 



Pneumatocyst (fig. 7, from above ; fig. 8, from below ; fig. 4, p, in vertical section). — 

 The float, filled with air, is campanulate or nearly spherical, with a central opening at 

 the basal pole of its vertical main axis. The diameter of this inferior opening measures 

 1 mm., and is about one-third as great as that of the float (3 mm.). The spherical 

 outer surface of the pneumatocyst is in close contact above with the exumbrella, beyond 

 the equator with the tentacular zone of the subumbrella. The inner cavity of the 



