REPORT ON THE DEEP-SEA KERATOSA. 



35 



very interesting species, is characterised by the fiat crusty form, representing a roundish or 

 subcircular disc, the peripheral margin of which bears a series of oscula or exhalent 

 openings, whilst the inhalent dermal pores are disposed on the flat upper face of the disc. 

 Psammina, therefore, closely approaches in structure those interesting Tetractinellidse 

 described by F. E. Schulze as Plakinidse, of which Plakina monolopha is a typical form, 

 because of its simple organisation. One of the three new deep-sea forms collected by 

 the Challenger is so similar to it, that it may be regarded as a Plakina monolopha, in 

 which the Tetractinellid spicules are lost and replaced by xenophya (calcareous shells of 

 the Globigerina ooze). This species (Psammina plakina) contains no symbiotic Hydroid. 

 The two other species are connected with a symbiotic Spongoxenia (probably Stylactis or 

 an allied genus); its reticular hydrorhiza, composed of brown anastomosing chitinous 

 tubes, is expanded horizontally in the medullar substance of the sponge, between the two 

 solid parallel skeleton plates of the cortical substance (upper and lower face). The 

 pseudo-skeleton in Psammina globigerina is composed of Globigerina shells ; in Psammina 

 nummulina of Radiolarian tests. The canals of the sponge are branched between the 

 meshes of the Hydroid tubes (h), and open together with these on the peripheral margin 

 of the disc. The isolated canals exhibit a distinct membrana propria (fig. 2D, c). 



Psammina plakina, n. sp. (PI. VII. figs. lA-lD). 



Habitat.— South Atlantic, Station 331 ; March 9, 1876 ; lat. 37° 47' S., long. 

 30° 20' W.; depth, 1715 fathoms; bottom, Globigerina ooze. 



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

 soft medullar substance between them, the former being composed of Globigerina shells, 

 the latter of maltha and a simple gastral cavity, covered by a single layer of flagellated 

 chambers. No symbiotic Spongoxenise. Several oscula on the peripheral elevated 

 margin. x 



Psammina plakina is a very remarkable form, which differs from the following typicaF 

 species of the genus in such essential points, that it may perhaps be better described as the 

 representative of a new genus, Psammoplakina discoidea. Tj^o small specimens were 

 observed forming white subcircular plates, the smaller 5 to 6 mm. in diameter, the larger 

 10 to 12, and 15 to 2 - 5 mm. in thickness. The internal structure is very similar to that 

 of Plakina monolopha, accurately described by Franz Eilhard Schulze. If we were to 

 suppose that the characteristic siliceous spicules of the Tetractinellid Plakina monolopha 

 were lost or dissolved and replaced by Globigerina ooze taken from the bottom of the 

 sea, then we should have the structure of Psammina plakina. 



The consistence of Psammina plakina is very hard and solid, not so rigid, however, 

 as in the similar following species. The subcircular disc is slightly convex on the lower, 



