58 THE VOYAGE OF H.M.S. CHALLENGER. 



The genus Porpalia and the following Porpema together make up the subfamily 

 Porpalidse, characterised by a strongly vaulted, lenticular or nearly spherical, umbrella, 

 which includes a mitriform or campanulate pneumatocyst ; the distal margin of the latter 

 being divided into radial lobes. The chitinous substance of this lobate pneumatocyst in 

 the Porpalidse is much thinner, more delicate, and softer than in the Porpitellidse (Por- 

 pitella and Porpita), sometimes it resembles crumpled tissue-paper. The numerous mar- 

 ginal tentacles in Porpalia are arranged in eight regularly disposed radial bunches, 

 whilst in Porpema they are equally distributed along the margin of the umbrella. 



The genus Porpalia is founded upon a new species, Porpalia prunella (PI. XLVIIL), 

 well-preserved specimens of which were found in the Challenger collection, taken in 

 the tropical Pacific (Station 222). The incomplete description of another species of 

 this genus, founded upon a single specimen, was given in 1829 by Eschseholtz, under the 

 name Porpita globosa. 1 He took this specimen in the tropical Atlantic, near the Cape 

 Verde Islands. The exumbrella is in this Atlantic Porpalia globosa much smaller, but 

 the tentacles larger than in our species from the Pacific. 



The phylogenetic position of Porpalia is clearly indicated by its morphological relation 

 to Disconalia on one hand, and to the Porpula larvae of the other Porpitidte on the other. 

 It may be regarded as the common ancestral form of this family derived from Disconalia 

 by the formation of a mouth on the distal ends of the gonostyles. 



Porpalia prunella, n. sp. (PI. XLVIIL). 



Habitat. — Tropical Pacific, north of New Guinea, Station 222 ; March 6, 1875; 

 lat. 12° 15' N, long. 146° 16' E. Surface. 



Umbrella (fig. 1, as seen from above ; fig. 2, in profile ; fig. 3, in meridional section). 

 — The umbrella is biconvex lenticular ; its diameter amounts to 4 or 5 mm. (without ten- 

 tacles and siphon). A deep submarginal ring-furrow separates the flat exumbrella from the 

 biconvex body, the superior face of which is more strongly vaulted than the inferior. 

 The equatorial diameter of the biconvex lens is twice as great as its vertical main axis. 



Exumbrella (figs. 1-3, uc). — The superior or apical face of the umbrella is rather 

 flat, with a shallow ring-furrow separating the central disc from the elevated peripheral 

 margin. The central disc exhibits an elegant pigment-star with eight dark brown rays. 

 Numerous stigmata are disposed in irregular rows between them. 



Limbus Umbrellte (figs. 1-3, um). — The free prominent border or limb of the umbrella 

 is as broad as the radius of the central disc of the umbrella, and therefore half as broad 

 as the equatorial radius of the lens. Its upper face is concave, the lower convex. The 

 thickened margin is reflected upwards, and contains a single series of the usual muci- 

 parous glands (compare above). 



1 1, p. 178, Taf. 16, fig. 4. 



