REPORT ON THE MONAXONIDA. 139 



of straight tylote spicules ; these spicules, just beneath the surface of the sponcre, are 

 collected into loose, irregular fibres, which break up into tufts of spicules on approachino- 

 the dermal membrane, (b) Main ; a very irregular, somewhat Halichondrioid reticulation 

 of spined styli, echinated by much smaller spined styli. There is no very distinct fibre, 

 though in places there appears to be a tendency towards the radiate arrangement 

 characteristic of Bowerbank's genus Microciona. 



Spicules. — (a) Megasclera ; (1) long, slender, straight or slightly curved, entirely 

 spined styli (PI. XXVII. figs. 6, &a), tapering very gradually to a fine point at the apex, 

 measuring about 0"42 by O'Ol mm. (2) Much smaller, usually straight, entirely 

 spined styli (PI. XXVII. figs. Qd, 6e), tapering gradually from base to apex and finely 

 pointed, size usually about 0'14 by 0*0065 mm.; distinguished from the preceding by their 

 position (echinating), their smaller size, and the fact that they are more strongly spined. 

 (3) Straight tylota (PL XXVII. figs. 66, 6c), with usually distinct, elongated, oval heads, 

 often narrowing towards the end; size about 0*28 by 0"0045 mm. (6) Mici'osclera ; 

 (1) tridentate isochelae (PL XXVII. fig. 6f), with slightly curved shaft; length about 

 0'037 mm. (2) Slender sigmata, usually much contort ; length about 0'056 mm. ((?) 

 (3) A few scattered, long, straight, hair-like, oxeote rhaphides ; length about 0"35 mm. 

 It is possible that these spicules may have been taken in by the sponge as foreign bodies, 

 and that they really belong to some of the Tedanise which occur at the same locality). 



Eejecting the long hair-like spicules as young forms or foreign, this sponge differs 

 from the type of the species only in the comparative slenderness of the megasclera. 

 The species comes very close in spiculation to Myxilla [Microciona) hihamigera, Waller.^ 

 In the latter, however, the megasclera are all smaller and the diactinal spicules without 

 heads (hastately pointed), while the isochelfe are described as being palmate ; the habit 

 and the arrangement of the main skeleton also differ in the two species. 



Locality. — Off the south-west coast of Patagonia. One specimen. 



Myxilla compressa, Ridley and Dendy (PI. XXVII. figs. 9, 9a, 9&, 9c, 9d, 9e). 



1886. Myxilla compressa, Ridley and Dendy, Ann. and Mag. Nat. Hist., ser. 5, voL xviii. p. 473. 



Sponge massive ("?), flattened. The largest piece in the collection is about 56 mm. 

 long by 25 mm. wide and 4 mm. thick. Colour in spirit yeIlow4sh-grey. Texture soft 

 and spongy. Surface smooth where the dermal membrane is still intact, but the sub- 

 dermal cavities show very distinctly through the transparent membrane and give to the 

 sponge a honeycombed appearance. Dermal membrane distinct, thin and transparent. 

 Oscida small and scattered, having their margins flush with the surface of the sponge. 

 Pores collected in groups — pore-areas — in those portions of the dermal membrane which 

 cover the subdermal cavities. The pores are round or oval and average about 0*2 mm. 

 in diameter. 



1 Jmirn. Quekett Micr. Club, vol. v. p. 1, pis. i., ii., 1878-79. 



