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



concave on the upper, side, the peripheral margin being curved somewhat upwards. The 

 convex basal (inferior) face as well as the concave free (superior) face are two parallel 

 thin white plates, composed of small Globigerina shells cemented together by a scarce 

 maltha. The medullar or intermediate plate enclosed between these two parallel dermal 

 or cortical plates is somewhat thicker than these, but much softer ; it is composed of 

 numerous small shells and fragments of Globigerina imbedded in a clear maltha, arid of 

 a very remarkable canal-system. The structure of this latter became evident, after 

 having dissolved the calcareous mass of the pseudo-skeleton by hydrochloric acid. Then 

 appeared a flat saccular or pouch-shaped central cavity, divided into irregular chambers 

 by mesodermal septa (figs. 1C, ID, g). From the upper face of this central sac arise 

 numerous lobate diverticles, which are beset with groups of flagello-chambers (k). These 

 open by small pores into inhalent canals (i), which descend vertically from the upper 

 face. From the periphery of the sac arise numerous excurrent canals, which open into 

 a few peripheral exhalent main canals, and these open free on the peripheral elevated 

 margin by oscula (o). About ten or twelve such peripheral oscula could be recognised as 

 larger openings, probably prolonged in the living sponge into prominent oscular tubes 

 or chimneys. 



The circulation of the water in Psammina plakina is evidently the same as in the 

 similar Plakina monolopha, the water entering by the inhalent pores of the concave 

 upper face of the disc, issuing by the exhalent oscula of the margin. Very remarkable 

 is the large simple gastral cavity, or the paragaster (fig. lC, g). This is divided in the 

 following species into numerous chambers, probably due to the development of the 

 symbiotic Spongoxenia (absent in Psammina plakina). 



In the middle portion of the discoidal body, in that portion of the mesodermal 

 maltha surrounding the basal flagello-chambers, are visible single scattered eggs, some 

 of which are in segmentation (PI. VII. figs. lC, ID, <?). Although badly preserved, the 

 egg-cells and their large clear nuclei were distinct. Their disposition is also similar to 

 that in Plakina monolopha. 



Psammina globigerina, n. sp. (PI. VII. figs. 2A-2D). 



Habitat.— Tropical Pacific, Station 220; March 11, 1875; lat. 0° 42' S., long. 

 147° 0' E.; depth, 1100 fathoms; bottom, Globigerina ooze. 



Sponge discoidal, subcircular, composed of two parallel hard cortical plates and a soft 

 medullar substance between them, the former being composed almost entirely of 

 Globigerina shells, the latter of maltha, with the canal-system and a network of 

 symbiotic Spongoxeniae. Exhalent oscula on the peripheral margin. Gastral cavity 

 chambered. 



