THE SPONGES. 



139 



f 



Tylodesma vestibularis, sp. nov. 



Plate 18, Figs. 8, 9; Plate 19, Fig. 1; Plate 22, Fig. 4; Plate 23, Figs. 1-3. 



Diagnosis. Sponge primarily incrusting, but it may so grow as to completely incorpo- 

 rate the substratum, thus appearing massive. Surface exhibits numerous vestibular 

 spaces, appearing as elongated cavities extending tangentially, and separated from the 

 exterior only by the dermal membrane ; opening at one end by an osculum. Transverse 

 diameter of such spaces, 1 to 4 mm. ; length, frequently 10 to 20 mm. Dermal membrane 

 in general riddled with pores. Color, light yellowish-brown. Sponge moderately firm, 



Spicules. Tylostyles, 630 /x x 16 /x to 240 p x 8 /x. Sigmata com- 

 Main skeleton consists of scattered tylostyles and irregularly 



but very brittle. 



monly 36 to 12 /x long. 



disposed short tracts of same spicule. In the superficial region numerous radial or ob- 

 liquely radial tracts are distinguishable, ending at the surface in projecting brushes. The 

 adjacent obliquely radial tracts, with their terminal brushes, are prolonged into the 

 vestibular membranes, there occupying an approximately tangential position. 



Station 340S, one specimen. 



The sponge (Fig. 1, Plate 19) is incrusting, below upon conglom- 

 erate, above upon the dictyonal framework of a Ilexactinella, which agrees, 

 in regard to the framework, with II labyrinthica mihi, and very probably 

 is this species. The thickness of the incrusting sponge, external to the 



conglomerate or Hexactinellid support, is about 1 



mm., or often less. 



grown 



through its interstices 



The conglomerate is in part a firm, solid mass ; in part, of a very loose 

 composition. Where the mass is loose, consisting of bits of shells, spines, 

 annelid tubes, and Polyzoa, the sponge has grown into all the crevices 

 between the component particles, and aids in holding them together. 

 Above, the sponge does not form a mere incrustation upon the surface 

 of the Ilexactinella skeleton, but has incorporated the latter, having so 



that the Hexactinellid framework is now 

 found in the interior of the sponge, along with the proper Monactinellid 

 skeleton (vide Fig. 4, Plate 22, and Fig. 1, Plate 23, sections vertical 



\ 



to the surface). Over a part of the surface the lobes of the supporting 

 Ilexactinella remain distinct, although they have been individually incor- 

 porated by the Tylodesma. But over most of the surface the primitively 

 incrusting sponge has filled up the gaps between the Ilexactinella lobes, 

 thus assuming the character of a continuous amorphous mass. This mass 

 is however, excavated internally by some large cavities, which probably 

 represent spaces between the Hexactijiella lobes. The Ilexactinella skeleton 

 is in a measure disintegrated, and along with it the massive part of the 

 Tylodesma has incorporated other fragments of a stony nature, most of 





