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



surface occupies less than half the area of the two opposite sagittal pairs (posterior and 

 anterior) ; these form a much broader oblique triangle with a prominent convex base. 



The superior or apical face of the disc is slightly convex, the inferior concave ; the 

 convexity is much stronger in younger specimens (fig. 4) than in older (tig. 3). 



The central chamber of the pneumatocyst (fig. G, ph) exhibits nearly the same 

 form as that figured in Rataria cristata (PL XLIV. figs. 8, 9). It is elliptical and dis- 

 tinctly octolobate. The deep frontal furrow proceeds near to the central chamber, so that 

 the octoradial ring is bisected by it ; its ventral half is composed of two anterior and two 

 left chambers ; its dorsal half of two posterior and two right chambers. Each of the eight 

 radial chambers possesses a branched trachea, arising from the periphery of its basal side. 

 The apical side of the octolobate ring, however, exhibits three stigmata only, placed near 

 the macrodiagonal, one subcentral, an anterior upon the left ventro-lateral, and a posterior 

 upon the right dorsolateral chamber. 



The concentric elliptical ring-chambers, which surround the octolobate central part of 

 the float, are sixty to eighty in number in the largest specimens ; those of the central and 

 those of the peripheral part are half as broad as the intermediate chambers placed between 

 them. Each elliptical ring-chamber is deeply bisected by the frontal furrow of the brachy- 

 diagonal. All elliptical ring-chambers communicate with one another by two opposite 

 openings or pneumothyrae, which are placed one on each pole of the sagittal axis (fig. 5,j>f/). 



The stigmata, or the external openings of the exumbrellar face of the float, are placed 

 in one straight line, very near the base of the vertical crest, and this line bisects the latter 

 at a very small angle (fig. 5, pe). There are in the largest specimen (besides the central 

 stigmata) about thirty to forty stigmata opposite in pairs ; these are situated, the half 

 in the right ventral quadrant of the disc (to the right of the basis of the crest) ; the other 

 half in the left dorsal quadrant (to the left of the basis of the crest). 



The tracheae, which arise from the inferior face of the disc, are not numerous. 

 Besides the eight above-mentioned tracheae, which usually arise from the octolobate ring 

 (fig. 6, pi), there is in some specimens another corona of sixteen tracheae, which arise from 

 the sixth or ninth ring, or between these. But in other species there is no regular arrange- 

 ment, a few scattered tracheae arising here and there from one of the middle chamber- 

 rings. In a few specimens I found an oblique double series of tracheae arising along the 

 frontal furrow, on both sides of it. The number, size, disposition, and branching of the 

 tracheae seem to be subject to many variations in this as well as in other species of the 

 Velellidae. Some tracheae are very long (about equal to the diameter of the centradenia), 

 and bear ten to twelve or more irregular and curved branches (fig. 10) ; others are much 

 smaller, and bear only a few short branches. The majority of tracheae run more horizontally 

 beyond the pneumatocyst, and finish in the glandular tissue of the centradenia ; but 

 a few larger tracheae pierce this latter, and enter partly into the exodermal wall of the 

 central siphon, partly in that of the gonostyles, where they end in their proximal half. 



