SILICIOUS AND HORNY SPONGES WILSON. 313 



in bundles. Here the somal oxea is again the most abundant spicule, 

 but there are also numerous dichotriaenes and some anatriaenes, the 

 cladomes of the triaene forms lying just below the cortex. Other 

 dichotriaenes, accompanied by anatriaenes and a few protriaenes, 

 along with an occasional large oxea, pass radially through the ster- 

 rastral layer of the cortex, constituting the very distinct radial 

 bundles of the cortex, which are of course best seen in the ecto- 

 chrote. A typical radial bundle in this region includes one or a 

 few dichotriaenes, the cladomes of which support the dermal mem- 

 brane, accompanied by a good many anatriaenes, and rather rarely 

 by a protriaene or two. Occasionally a large oxea forms the axis of 

 a bundle, instead of a dichotriaene. The radial bundles of the ecto- 

 chrote are 100-350 jx. apart. The skeleton of the cortex and peripheral 

 choanosome is the same on the two surfaces of the sponge. 



/Spicules. — 1. Dichotriaene (pi. 46, figs. 3, 5). Rhabdome 3-1 mm. 

 long, 70 y. thick near cladome; protoclad 100 \l long; deuteroclads 

 150 [x long; measurements are for the superficial or dermal spicules. 

 The cladomes of some spread out paratangentially just below the 

 sterrastral layer; these are somewhat smaller than the superficial 

 ones. The cladomes of others lie in and support the dermal mem- 

 brane, dividing it up into imperfectly separated areas, which lack 

 uniformity. The rhabdome of a superficial dichotriaene lies of 

 course in an ectosomal partition (trabecula), and a clad may lie 

 within the limits of a partition. Most of the clads extend out how- 

 ever into the thin sheets of membrane between the partitions. Even 

 here sometimes pores can be seen directly over a clad, indicating that 

 between the clad and the surface there are minute cavities. 



2. Somal oxea, 3 mm. long, 50 y. thick; smooth, tapering, usually 

 slightly curved; occasionally taking the shape of a style (stron- 

 gyloxea). 



3. Anatriaenes of the radial bundles (pi. 46, fig. 3). Rhabdome 

 3 mm. long, 16 [x thick below cladome, clads 70-80 [l long. 



1. Protriaenes of the radial bundles (pi. 46, fig. 3). Rhabdome 

 2-3.5 mm. long, 8-24 \k thick : clads 40-90 \i long. The cladome some- 

 times has four rays, these of unequal length. The large protriaenes 

 sometimes seen in the interior are probably stages in the develop- 

 ment of the dichotriaene. 



5. Ectosomal oxea (pi. 46, fig. 3), 370-500 by 6-8 p.. Smooth, 

 evenly tapering; straight or slightty curved. Perforating, and 

 slightly projecting from, the dermal membrane radially or obliquely, 

 or tangential in it; singly or in loose sheaves; abundant. Abundant 

 also in the peripheral choanosome. 



6. Sterraster (pi. 46, fig. 3). A flattened ellipsoid; face 124 by 

 104 jx; thickness 80 jx. Sterrastral layer of cortex about 1.2—1.5 mm. 

 thick. Abundant sterrasters scattered through choanosome. 



