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



tentacles. The first species observed was taken by Eschscholtz in the tropical Pacific, 

 and described in 1829 as Porpita ccerulea. 1 It is probably the species of which Huxley 

 afterwards gave a very accurate anatomical description. Another species of this genus, 

 also taken in the tropical Pacific (near the Marianne Islands), is Porpita radiata of 

 Brandt. 2 I have been able to compare the excellent (hitherto unpublished) figure and 

 description left by its discoverer, Mertens. Two specimens of a third species, described 

 in the following pages, were found by me in the Challenger collection, from Station 244. 

 It exhibits in several points a remarkable similarity to the interesting deep-sea Medusa, 

 Pectanthis asteroides. 3 I call it therefore after this Trachynemid. 



The phylogenetic origin of Porpitella is to be found in Porpalia. It may be 

 derived from this ancestral form by flattening of the highly vaulted umbrella and the 

 pneumatocyst, which thus become more or less discoidal. 



Porpitella pectanthis, n. sp. (PL XLVL). 



Habitat. — Station 244, Northern Pacific, between Japan and Honolulu ; June 28, 

 1875 ; lat. 35° 22' N., long. 169° 53' E. Surface. 



Umbrella (fig. 1, from above; fig. 2, from below; fig. 3, meridional section). — The 

 disc is a biconvex lens, the horizontal diameter of which amounts to 12 or 15 mm., 

 and is three times as great as its vertical axis (4 to 5 mm.). The meridional section 

 (fig. 3) demonstrates that the larger superior half of the umbrella is occupied by the 

 discoidal pneumatocyst (p), the smaller inferior half by the lenticular centradenia (uc). 



Exumbrella (fig. 1). — The superior or apical face \ of the umbrella is slightly convex, 

 and exhibits an elegant radial striation, crossed by numerous concentric rings. Sixteen 

 more prominent ribs bear a number of conical papillse, and between these numerous 

 more delicate, also denticulate, ribs radiate from the centre. The stigmata open at the 

 apex of the conical papillse. An elegant pigment-star with sixteen brown rays indicates 

 the course of the main ribs. 



Limbus Umhrellee (figs. 3, 7, um). — The free margin of the umbrella is prominent at 

 the periphery of the lenticular body, and about one-third or one-fourth as broad as the 

 length of its radius ; it is circular, divided by sixteen slight incisions into sixteen flat 

 marginal lobes (fig. 1). Its thickened edge includes a series of marginal glands (fig. 7, us). 



Subumbrella (fig. 2). — The inferior or basal face of the umbrella is slightly convex, 

 and divided into four different parts of nearly equal breadth. The central part is 

 occupied by the large sterile central siphon ; this is surrounded by three circular 

 concentric zones ; the first zone is occupied by sixteen smaller sexual siphons (bearing 



1 1, p. 179, Taf. 16, fig. 5. 2 25, p. 40. 



3 Zool. Cliall. Exp., part xiL p. 20, pis. vii., viii. 



