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



employed for a Sponge) comprises those Velellidse which differ from the true Velella by 

 the broad lobate margin of the umbrella and by the double or multiple corona of 

 tentacles ; usually also by the quadrangular form of the umbrella, which in the former is 

 more elliptical. Although a sharp boundary line between the true Velella (s. str.) and 

 Armenista cannot be drawn, the differences of the extremely divergent forms are very 

 striking ; and when the intermediate forms are neglected, they represent two widely 

 different genera. The elliptical umbrella with integral border of the mantle, and the 

 simple corona of tentacles, brings the true Velella nearer to the ancestral Rataria. On 

 the other hand, the larger Armenista, with the broad lobate border of the mantle of the 

 quadrangular umbrella, and the double or multiple corona of tentacles, is a more recent 

 and more differentiated form. To this genus belong probably the following species : — 



(1) Armenista sigmoides, n. sp., from the Southern Tropical Atlantic, figured in 



PI. XLIIL, and taken by the Challenger at Station 346. 



(2) Armenista mutica, Lamarck, from the North-western Atlantic, very good 



figures of which were published in 1883 by Alexander Agassiz (57). 



(3) Armenista antarctica, Eschscholtz (1), and 



(4) Armenista indica, Eschscholtz (1), from the Indian and Antarctic Oceans, and 



the Cape of Good Hope. 



(5) Armenista lata, Chamisso (21), from the Northern Pacific. 



(6) Armenista lobata, n. sp., from the Southern Pacific. 



Armenista sigmoides, n. sp. (PI. XLIIL). 



Velella sigmoides, Hkl., 1881, MS. 



Habitat.— Station 346, Tropical Atlantic; April 6, 1876 ; lat. 2° 42' S., long. 14° 47' 

 W. Surface. 



Umbrella (fig. 1, from above; fig. 2, from below; fig. 3, half lateral, half superior 

 view). — The umbrella is nearly rectangular, with rounded edges, about once and a half 

 or twice as long as broad ; the longitudinal or sagittal diameter of the horizontal disc 

 is in the largest specimens 80 to 90 mm., the transverse or frontal diameter 30 to 

 40 mm. This latter about equals the height of the triangular vertical crest or sail. 



Exumbrella (fig. 1). — The superior or apical face of the umbrella is divided into the 

 broad lobed mantle-border and the colourless pneumatic disc, both separated by a sharjD 

 line which is not parallel to the margin of the border. The vertical diagonal crest 

 divides the exumbrella into two equal halves, an antero-dextral and a postero-sinistral. 

 When we regard the disc in profile from the broad side (its major axis perpendicular to 

 the axis of vision), the crest or sail runs from our proximal and left to our distal and 

 right side ; but seen from behind, from the smaller side (the major axis of the disc 



