THE SPONGES. 121 



4 millim. apart. Dermal skeleton — extremely loose tracts of irregularly par- 

 allel spicules, 3 or 4 spicules broad, surrounding roundish or polycronal 

 areas from .18 to .28 millim. in diameter." 



I have examined the " Challenger " specimen of P. variabilis var., and add 

 the following note. The body of P. variabilis crassa is much harder and much 

 less cavernous, and the fibres of the main skeleton are much thicker than 

 in the "Challenger" specimen. The surfaco^ in general, of the " Challenger" 

 sponge is distinctly reticulate to the eye. The obvious character of the 

 reticulum, as compared with the condition in P. variabilis crassa, is due to 

 the relative slenderness of the fibres and large size of the meshes. Over 

 some parts of the surface, however, the reticulum is very indistinctly de- 

 veloped, the fibres being thicker and the meshes smaller than over the 

 general surface. In such places fibres about as thick, and meshes about 

 as small, as in P. variabilis crassa may be found. 



PetrOSia Similis Ridley and Dendy. 

 1887. Petrosia similis Ridley and Dendy, 1887, pp. 9-12, Plates II. III. 



Petrosia similis densissima, subsp. nov. 



Plate 17, Figs. 7, 10; Plate 21, Figs. 4, 5. 



Diagnosis. Sponge irregularly lobate ; lobes short, subcylindrical branches, or merely 

 rounded protuberances. Hard, almost stony. Surface appears finely reticulate and 

 smooth to the eye. Oscula, 2-3 mm. in diameter, at or near the free ends of the lobes, 

 leading into cloaca-like cavities. Poves in meshes of the dermal reticulum, 1 to 5-6 pores 

 in a mesh. Oxea, 220 yu, x 16 yu,. Skeletal fibres compact, consisting of many rows of 

 spicules with but little spongin ; 80-180 /a thick. Very few spicules in the meshes of the 

 skeletal reticulum. In the superficial region of the body, main skeletal reticulum regular, 

 consisting of radial fibres with connectives; meshes here 170-250 jx in diameter. In the 

 interior, main skeletal reticulum irregular, with meshes 200-500 /x in diameter. Dermal 

 reticulum merely the outermost layer of the main skeleton; fibres, 170-250 /* thick; 

 meshes, 170-250 jx in diameter. 



Station SJf05, 1 specimen. 



The specimen is an irregular lobate mass, attached below to white con- 

 glomerate, upon which for a short distance it spreads out in an incrusting 

 fashion. The mass is 40 mm. high, somewhat flattened ; width of the 

 flattened faces about 40 mm. In Fig. 7, Plate 17, one of the flattened 

 surfaces is shown. Some of the lobes are well marked, though short, 

 subcylindrical branches; others, mere rounded protuberances. The trans- 

 verse diameter of the branches, and the thickness of the whole mass in 



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