SPONGITDA. 623 



longer and has more bends than the stout form, which could thus 

 only have been derived from it by fission or retrogressive absorption, 

 methods unknown, so far as I am aware, in the normal development 

 of siliceous sponge-spiculcs. 



46. Spirastrella transitoria. (Plate LTV. figs, q, q'.) 



Sessile, incrusting. Surface broken up by slight intercrossing 

 ridges into very shallow angular areas 1 to 2 millim. wide ; surface 

 between and over ridges subglabrous. Consistence in spirit tough, 

 elastic ; colour pale pinkish brown. 



Main skeleton chiefly composed of dense fascicles of the skcleton- 

 spicule, with the points radiating outwards, set at various angles to 

 the surface, viz. from right angles to a horizontal position ; the 

 points of the bundles frequently project slightly beyond the surface. 

 Sarcode dense ; that of surface subtransparcnt, dark greenish yellow ; 

 of subjacent tissues very pale yellow, transparent. 



Spicules : — (1) Skeleton spinulate, straight or nearly so ; head 

 spherical, neck distinct ; head and centre of shaft of about the same 

 diameter, viz. -016 millim. ; shaft tapering to sharp point from about 

 7 diameters from apex ; length of spicule about '9 millim. (2) Spini- 

 spirular, extremely concentrated, composed of only one entire bend ; 

 spines numerous, closely aggregated, strong and sharp ; shaft about 

 ■004 millim. thick ; spines - 004 millim. long; length of spicule, 

 including spines, '016 millim. 



Hab. Darros Island, Amirante group, 22 fms. ; bottom broken 

 coral. 



This species is represented by a specimen of about 1 square inch 

 in superficial extent, covering and following the inequalities of a 

 small mass of shells and Polyzoa ; the thickness varies from about 

 •5 to 2 millim. It appears to be most nearly related to the form, 

 termed by Mr. Carter (Ann. & Mag. ST. H. 1882, ix. p. 352) 

 " Spirastrella cunctatrix, variety," from Mauritius ; but this form is 

 stated to be white, to have a spinispirular with two bends, of a length 

 of about '036 millim. It differs from S. cunctatrix, Schmidt, in the 

 shorter spinispirular, and the globular, not oval, head of the spinulate. 

 In S. transitoria we have the spinispirular almost in the form of the 

 stellate, with which Schmidt (Spong. Atl. Geb. p. 5) and Carter 

 (op. cit. 1879, iii. p. 355) consider it to be homologous. 



47. Spirastrella punctulata. (Plate LIV. figs. p,p.) 



" Suberites ? sp. undescribed, Mauritius," Carter, Ann. 8r Mag. N. H. 

 1882, ix. p. 352. 



Elongated, subcorneal. Vents single or few, terminal, oval, about 

 2 millim. in greatest diameter. Surface obscurely nodose, and 

 covered besides on the upper parts of the sponge with a minute, but 

 close and regular pitting, giving the appearance of shagreen ; pits 

 and intermediate elevations low, each about -3 millim. in diameter ; 



