412 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



localities Christmas Island and the Mergui Archipelago. Hent- 

 schel, 1912 (p. 400), records the species from the Aru Islands but 

 remarks that it is not sharply delimited from P. diffusa (Ridley). 

 Hentschel adds (p. 402) that any division of the intricately varying 

 Pachychalina forms into species and varieties has but little per- 

 manent value, a conclusion for which much may be said to-day as in 

 O. Schmidt's time. 



PACHYCHALINA FIBROSA, var. GRACILIS, new variety. 



Station D5136, two masses perhaps originally united, each com- 

 posed of several long, slender, subcylindrical, branching shoots, some 

 of which reach 300 mm. in length. Similar specimens were taken at 

 D5145. The shoots fuse with one another in an irregular, accidental 

 fashion. Surface covered with large and prominent spines. Di- 

 ameter exclusive of spines commonly about 6 mm. ; but the shoot may 

 be distinctly flattened, diameters in a typical case 7 mm. and 4.5 mm. 

 Spines variable in size, 2-5 mm. high, mostly 2-5 mm. apart. 

 Oscula 2 mm. or somewhat less in diameter, in a single row along a 

 shoot, usually 5-6 mm. apart. A reticulum of yellow fibers shows 

 plainly through the dermal membrane; nodes of the reticulum dis- 

 tinct, meshes 3-5 sided and about 1 mm. wide. Sponge light brown ; 

 firm, flexible, but easily cracked in bending. 



Main skeleton a reticulum of stout fibers consisting of spongin 

 cored by multiserially arranged megascleres. The reticulum has 

 considerable regularity in that radial fibers and connectives are dis- 

 tinguishable, the latter commonly transverse and the meshes more or 

 less rectangular. 



Radial fibers 120-175 jx thick, core of spicules one-third to one- 

 fourth total thickness of fiber. Connectives very similar but in gen- 

 eral slightly slenderer and with a somewhat thinner core of spicules 

 in which the individual spicules are less densely packed. Meshes of 

 the reticulum 500-900 [/. wide. 



Spicules between the fibers of main skeleton not scattered as in 

 P. fibrosa but surrounded and united by spongin, thus forming fine 

 secondary reticula which occupy the meshes of the main skeleton. 

 Fibers of the secondary reticula only about 8-20 pt. thick, unispicular 

 or containing 2-3 rows of spicules, spongin relatively abundant; 

 width of mesh commonly near the length of a spicule but also greater. 



Dermal skeleton essentially a fine reticulum of very slender mostly 

 unispicular fibers relatively rich in spongin; width of meshes, which 

 are rounded, about the length of a spicule or less. This fine reticu- 

 lum as a rule overlies and is distinct from the superficial (tangential) 

 fibers of the main skeleton which show beneath it and at first sight 

 appear to divide it into a system of large meshes. And indeed some 



