SILICIOUS AND HORNY SPONGES— WILSON. 283 



45 mm. high. A broken edge in one region is interpreted as the 

 lower. The remaining and larger part of the edge is rounded and 

 uninjured, evidently free. The lamella is somewhat curved, and 

 the curvature is irregular, but in the region of the free margin one 

 surface is slightly convex, the other concave. 



The two surfaces of the lamella are alike in that each is covered 

 with abundant small apertures, often about 100 \i in diameter, but 

 ranging from 60 to 250 [X, the differences in size perhaps being due 

 in part to different degrees of closure. Typically there is one aper- 

 ture in each mesh of the dichotriaenal reticulum. Each leads into 

 a canal which penetrates the interior of the sponge. Doubtless one 

 surface is the pore, the other the oscular surface, but anatomically 

 they are not distinguishable. 



Spicules. — (1) Dichotriaenes (pi. 45, fig. 9), alike on both sur- 

 faces, cladomes at the surface, rhabdomes radial. The clads overlap, 

 forming a rather irregular reticulum. Protoclad 100 by 70 jx; deu- 

 teroclads 360 jx long, strong; rhabdome 450 by 75 jx, tapering, pointed. 

 A few dichotriaenes occur in the interior, where they occupy no 

 constant position with respect to the sponge surfaces. 



(2) Large oxeas, slightly curved, commonly about 2 mm. by 70 jx; 

 passing radially or obliquely through the sponge, the two ends pro- 

 jecting on opposite surfaces; also scattered tangentially and 

 obliquely in interior, some of the internal spicules reaching 3.5 mm. 

 by 80 u. 



(3) Long, slender oxeas, about 850 by 8 fx; spicules projecting, 

 obliquely radial, especially round the rhabdome of the triaenes; 

 inequiended, the outer slenderer end very thin and like a whiplash, 

 often broken off. Alike on both surfaces. 



(4) Small oxeas (pi. 45, fig. 8, a), classed as choanosomal mi- 

 croxeas, 160-220 by 6 tx, with slight centrotylote ringlike enlarge- 

 ment ; very abundant in parenchyma. Spicules offering a transition 

 to the dermal microxea can be found on searching, but they are very 

 few. 



(5) Dermal microxeas (pi. 45, fig. 8, Z>), 32-40 by 4 [x; spicule 

 slightly centrotylote; the two halves very slightly bent upon one 

 another, the spicule thus presenting an angular projection at the 

 middle; lying tangentially in the dermal membrane of both sur- 

 faces, forming a thin but dense crust. 



(6) St rept asters (pi. 45, fig. 8, c) of the metaster type, varying 

 toward the amphiaster and plesiaster. Axis short and slightly 

 curved, rays long and slender; rays commonly grouped near the 

 ends of the axis, 3 to 6 at each end. The larger spicules approach 

 the plesiaster type. In end view the spicules appear as oxyasters. 

 Total length of spicule 16-30 \i. In the walls of canals and scat- 

 tered in parenchyma. 



Holotype.— Cat. No. 21290, U.S.N.M. 



