104 LANUGONYCHIA FLABELLUM. 



lamella about 60 mm. long, 40 mm. broad, and 1 mm. thick, from the sharpest 

 angle of which arises a uniformly curved stalk about 90 mm. long and 2-3 mm. 

 thick. Whether the complete sponge, of which the specimen formed a part, 

 was also fan-shaped, is hard to say. It may have been calyculate or even tubu- 

 lar, but it certainly was thin-walled and stalked. 



The colour in spirit is rather dark reddish brown. 



General structure. The lamellar body is reticulate in structure (Plate 13, 

 fig. 14), composed of a network of bands, mostly 0.2-0.5 mm. thick, which enclose 

 round meshes, 1 mm. wide. These meshes are partly covered by remnants of 

 superficial membranes. 



The skeleton of the stalk (Plate 13, fig. 7) consists chiefly of longitudinal 

 beams 20-90 ^ thick (usually 40-80 m), about equally far apart, and joined at 

 frequent intervals by short transverse bars. The latter are thickened, and 

 trumpet-shaped at the base. The meshes of the whole ladder-like network 

 formed by the beams and bars are rounded, usually oval, 35-210 fi long and 25- 

 90 n broad. Most of the longitudinal beams are rhabds; some appear to be 

 elongated rays of pentactines and hexactines. 



The interior of the lamellar body is occupied by rhabds and great numbers 

 of onychhexasters, and large and small regular discohexasters. Plumicomes 

 and a few irregular discohexasters with primary and secondary end-rays have 

 also been found in it. Besides these spicules several large amphiasters have been 

 observed. These are, however, most likely foreign to the sponge. Very proba- 

 bly small hexactines also occur in the choanosome. I am not, however, certain 

 about these spicules ; the ones observed may in truth have been gastral or dermal 

 and brought down into the choanosome accidentally. Below the surface pentac- 

 tines are met. The superficial (dermal and gastral) skeleton consists of hexac- 

 tines and pentactine to monactine hexactine-derivates with only five to one 

 properly developed and one to five reduced rays, which latter appear as short, 

 terminally rounded protuberances of the centre of the spicule. The tetractine 

 forms are stauractines; the diactine forms are mostly centrotyle amphioxes; a 

 few of them appear compass-shaped. On one side of the lamella true hexactines, 

 with all six rays fully developed, greatly predominate, spicules with only five or 

 four fully developed rays (pentactines and stauractines) being rare, and spicules 

 with only three or still fewer (triactines to monactines) absent altogether. On 

 the other side of the lamella hexactine-derivates with fewer than six fully 

 developed rays are more frequent than true hexactines and here also triactine to 

 monactine forms with only from three to one fully developed rays are frequently 



