406 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



Holotype.— Cat. No. 21337, U.S.N.M. 



This species adds one more to the list of Petrosias in which the 

 spicules range from large to very small ones. Besides the Mediter- 

 ranean P. dura (O. Schmidt) and the North Atlantic P. crassa 

 (Carter), which is sometimes large and with crateriform cavity 

 (Topsent, 1904, p. 241), P. strongylata Thiele (1903, p. 938) from 

 Ternate, P. truncata, var. aruensis Hentschel (1912, p. 402) from 

 the Aru Islands, P. nigricans Lindgren (1898, p. 287) from Java, 

 and P. nigricans, var. irregularis Hentschel (1912, p. 405) from the 

 Aru Islands, may be noted. 



The differences offered in the skeletal framework by the three 

 specimens of this species (type and variety) have some interest. 

 They are, in all probability, individually acquired, not hereditary, 

 differences. Assuming this, it becomes worth while to formulate 

 the question whether some of the individual peculiarities are not 

 adaptive responses (of growth and differentiation) to mechanical 

 stresses and strains, such as those exerted by water currents and 

 the weight of the whole body — as, for instance, in the case of 

 bones, where there seems to be a very considerable power of adapt- 

 ing the arrangement of bone lamellae to the mechanical needs of 

 the individual case? Experimentation on this point is doubtless 

 feasible. 



PETROSIA SIMILIS Ridley and Dendy, var. GRANULOSA, new variety. 



Plate 41, fig. 7. 



Petrosia similis Ridley and Dendy, 1887, p. 9. 



Several elongated, subcylindrical, more or less flattened pieces 

 from Station D5141. Three measure, each, about 100 mm. long; the 

 fourth is shorter; transverse diameters in typical regions, 20 and 

 17 mm., 30 and 20 mm., 35 and 25 mm. The pieces widen termi- 

 nally, where one is slightly lobate. Sponge heavy, firm and incom- 

 pressible, but not stony; dark reddish brown, owing to the pres- 

 ence throughout the body of quantities of cells containing reddish 

 brown granules. 



Surface appears smooth to the eye; uniformly dense or showing 

 the ends of very small canals, a fraction of a millimeter in diam- 

 eter, distributed generally about 2 mm. apart. With a lens the sur- 

 face, where uninjured, is seen to be distinctly, though minutely, 

 conulose, radial skeletal fibers entering into and supporting the 

 conuli. Pores closed. Oscula abundant, mostly about 3 mm. in 

 diameter. They exhibit an imperfectly developed bilateral distribu- 

 tion, in that, although scattered ones occur, they tend to be con- 

 fined to the opposite and narrower sides of the sponge, thus 

 forming somewhat vaguely marked rows. Interior of sponge com- 

 pact, canals small; ratio of soft tissue to skeleton fairly high. 



