116 PkOCEEDINGS of the national museum vol.81 



worthy for external simulation of these two sponges. We must con- 

 clude that the remarkable structure is in a large part an ecological 

 adaptation. This should not be interpreted that this is some common 

 shallow-water sponge that has lightly assumed such shape because of 

 its surroundings, but that this species has by selection or some other 

 determiner come to have this structure. It is remarkable further 

 in that it is the slightly convex side that is oscular; usually in con- 

 cavo-convex sponges the reverse is true. This very unusual cir- 

 cumstance is also reported for Xestospongia (Petrosia) coralloides 

 Dendy (1924, p. 325). It may be a generic tendency. X. diprosopia 

 is remarkable for this character, which it shares with coralloides, 

 and for the large size of its spicules, by which it may be distinguished 

 from that species and most others. 



XESTOSPONGIA VANILLA (de Laubenfcls) 



Ealiclona va/nilla de Laubenfkls, 1930, p. 28. 



Holotype.—V.S.'NM. No. 21452; B.M. No. 29.8.22.45. 

 Type locality. — Pacific Grove, Calif., July, 1925, collected by me. 

 This is one of the most abundant sponges in central California, oc- 

 curring usually on the under side of bowlders in the lower half of 

 the intertidal zone. 



Description. — Shape, encrusting. Size, up to 1 cm thick, spread- 

 ing laterally indefinitely. Consistency, stony hard. Color in life 

 and when preserved, white or very pale yellowish drab. Oscules, 

 round, often with raised rim, diameter 1 to 1.5 mm, distance apart 



about 1 cm. Pores, approximately 

 lOOfj. in diameter. Surface, super- 

 ficially smooth. 



Ectosomal specialization, vague or 

 lacking. Endosomal structure, a re- 



F-iGUBE 70.— Xesto.spongia vanilla (de fip„l„fi^,. nf cimk in i frroiind snh- 



stance in which the spicules are so 

 densely packed that no pattern can be discerned in their arrange- 

 ment. The canals, however, are usually either perpendicular or 

 parallel to the surface and often meet at right angles, so that they 

 make a s^^mmetrical pattern. 



Principal spicules, oxeas (fig. 70) ; size, 11/x by 150ju to l^fji by 160/^. 



Remarks. — Petrosia was erected by Vosmaer (1885, p. 338) for 

 Reniera dura of Schmidt (1862, p. 76), to replace Schniidtia of 

 Balsamo Crivelli; that name being preempted. He lists various 

 forms and specifies stony consistency, spicules crowded together, 

 oxeas, strongyles, and rarely styles. Ridley and Dendy (1887, p. 9), 

 define Petrosia thus : 



Sponge usually hard or even stony ; generally with numerous, well-defined 

 large oscula. Skeleton more or less confused ; spicules oxeote to strongylote. 



