SILICIOUS AND HORNY SPONGES WILSON. 471 



LITHISTIDA IXCERTAE SEDIS. 

 BRACHIASTER, new genus. 



BRACHIASTER SIMPLEX, new species. 



Plate 51, figs. 8, 9. 



A macerated lithistid was taken at station D5162, the desmas 

 of which are of such a novel and interesting character that it is 

 desirable to name the sponge, although, owing to the loss of the 

 microscleres, it can not be placed in the classification. 



Sponge lamellate, approaching a triangular shape, about 70 mm. 

 wide, 8 mm. thick. The entire free edge is rounded and appears to 

 be natural. One surface, slightly convex, is heavily incrusted with 

 sand. The opposite surface is slightly concave and is cleaner. 

 Sponge stony and yet in some slight degree flexible. 



Only the desmas remain. They are tetracrepid and form a coher- 

 ent skeleton, but on boiling in caustic potash or nitric acid a good 

 many, in more or less perfect condition, separate out. In hydro- 

 fluoric acid, of course, they separate out completely. The young 

 forms, which have not yet entered into articulation, are compara- 

 tively abundant. 



The young forms are simple dichotriaenes (pi. 51, fig. 9a), or 

 dichomesotriaenes (fig. 9b), or more complicated spicules (fig. 9e) 

 in which the primary clads may bear three branches instead of two, 

 the secondary clads again branching dichotomously or trichoto- 

 mously, and in which one or both of the axial rays branch in similar 

 fashion. 



The adult desmas are obviously dichotriaenes or dichomesotriaenes 

 in which the secondary clads and axial rays break up each into two 

 or three terminal branches (pi. 51, fig. 8). The facets for articula- 

 tion (a. s.) with other desmas are terminal or lateral on the branches 

 or in the angles between branches. Some irregular variants of the 

 type occur. 

 ' Holotype.— Cat. No. 21254. U.S.KM. 



The following macerated and unidentifiable lithistids were taken : 

 At station D 5172, a shallow cup-like sponge 70 mm. in diameter. 

 Desma monocrepid. 



At station D 5218, two specimens evidently of same species, the 

 larger 80 mm. high. Sponge excavated by an axial cloaca of con- 

 siderable size. Outer surface of skeleton labyrinthine, divided into 

 trabeculae and lobes about 2-4 mm. wide by somewhat narrower 

 meandering channels and spaces. Desma monocrepid with rosette- 

 like tubercles resembling those of Leiodentuit'nun pfei ferae. 



