Classification of the Spongida. 185 



indistinctly microspined ; or large and sparsely, but evidently, 

 spined, especially towards the extremities : sometimes absent 

 altogether. Forms already mentioned. 



Family 4. Pachasteellida. 

 Group 17. Pachastrellina. 



Incrusting, passing into crevices of the rock or other hol- 

 low cavities, massive, sessile, thick, flat, or semiglobular and 

 sessile. Structure nbreless, confused — that is, with little or no 

 apparent regularity of the spicules with which the areolar 

 structure of the body is densely charged, and no nucleus. 

 Surface plane, uniformly harsh asperous, with no distinct cor- 

 tical layer of any kind, saving the thickened dermal sarcode. 

 Colour light grey or dark grey, passing into soot-black, espe- 

 cially on the surface. Vents scattered singly or in groups on 

 the surface. Pores in the smooth interstices of the dermal 

 sarcode, situated between the projecting ends of the spicules. 

 Branched excretory canal-system well developed. Texture 

 asperous. Spicules of two kinds, viz. skeleton- and flesh- 

 spicules. Skeleton-spicule of three forms, viz. : — 1 , large trira- 

 diate, with the shaft or fourth arm aborted or obsolete, or tri- 

 radiate, with the fourth arm produced, quadriradiate ; arms 

 smooth, conical, pointed simply, or furcated once or twice 

 irregularly ; 2, smaller, consisting of a three-armed shaft with 

 the arms regularly furcate, and spreading almost horizontally 

 or perpendicularly to the shaft ; 3, long, simple, acerate, 

 more or less curved. Flesh-spicules stelliiorm sinuous and 

 spined, or bacillary and spined ; or ellipsoidal, inflated in the 

 centre, and smooth (skittle-like) ; or acerate and more or less 

 inflated, curved, and microspined ; or acerate, smooth, or 

 simple " tricurvate ; " or globostellate, with the rays rounded 

 tuberculiform, the whole resembling the siliceous balls of the 

 Geodina. 



N.B. The once or twice irregularly furcated extremities of 

 the larger triradiate spicule in Pachastrella abyssi, Sdt., leads 

 to the more elaborate furcation &c. in the following group, 

 viz. the Lithistina. 



Group 18. Lithistina. 



Massive, substony, excavated ; dish-shaped, circular, withjthick 

 undulating wall, stipitate ; or vasiform,with more or less thick 

 undulating wall, which, in some species, may be so plicated 

 meandrinately as to almost fill the centre, and thus produce the 

 appearance of a double flower. Structure more or less confused, 



