EEPORT ON THE MONAXONIDA. 161 



Skeleton. — A reticulation of well developed horny fibre, cored and echinated by 

 subtylostylote spicules of one kind only. 



Spicules. — (a) Megasclera ; (l) smooth, fusiform subtylostyli, sharply pointed and 

 constricted before the head, size about O'l by 0*0048 mm., in the fibre and echinating it 

 and scattered. (2) Very long, thin, hair-like spicules, in which it is extremely difficult to 

 make out the nature of the ends, (1) cylindrical, subtylostylote sometimes ; length about 

 0'17 mm.; scattered, (b) Microsclera; at first we thought that this species was sharply 

 marked off from EcMnoclathria carteri, nobis, by the absence of microsclera, but since 

 then we have seen a very few, faintly discernible, hair-like isochelse, about 0'0126 mm. 

 long, apparently on the verge of disappearance. 



This species we have been enabled to identify through our examination of Mr. Carter's 

 dried types in the British Museum.^ His description (loc. cit.), seems to us to be 

 misleading ; it is possible that his account of the spiculation was taken from some 

 other specimen than the two which we have examined (viz., his numbers 554 and 

 208 bis). 



The species comes extremely close to our Ecliinoclafhria carteri, but for the sake of 

 convenience we separate them ; they are distinguished by the fact that EcMnoclathria 

 carteri possesses abundant, distinct isochelse,' and well-developed, long, slender subtylo- 

 stylote spicules (scattered) ; the proportions of the spicules also diflfer in the two and 

 the typical external forms of the two species are very distinct [cf. figs. 3, 4 and 5 

 on PI. XXXI. ). That the two species are connected by a series of intermediate forms 

 there can be little doubt, but for the sake of convenience we at present keep them 

 distinct. 



The Challenger obtained six specimens of Echinoclatlma favus, all of which are 

 based upon Pecten shells ; the inhabitants of the shells were evidently living when the 

 sponges were dredged. This is a very interesting fact, for possibly we have here a case 

 of commensalism. It is difficult to understand how a Pecten can swim about (in the 

 manner stated of species of this genus by our conchological authorities) with a sponge 

 of the size represented in PI. XXXI. figs. 4, 5, attached to it, but the animal is still 

 in the shell and in good condition; possibly the streams of water engendered by the 

 sponge, bringing constant supplies of food, render motion on the part of the Pecten 

 unnecessary. 



Xoca%.— Station 162, AprU 2, 1874; lat. 39° 10' 30" S., long. 146° 37' 0" K; off 

 Moncoeur Island, Bass Strait; depth, 38 fathoms; bottom, sand and shells. Six 

 specimens. 



1 Mr. Carter, he. cit, says—" There are several specimens of it in the British Museum, mostly under four inches in 

 their greatest diameter, of which Nos. 554 and 555, each registered 59. 10. 7. 106, may be mentioned as massive forms, 

 and No. "208 6is" registered 37. 5. 13. 36. &c., as more or less digitate forms." 



2 Mr. Carter mentions no isochelae in EcMnoclathria favus, neither have we been able to find them in his tlried types, 

 but such delicate microsclera may be very readily overlooked in examining dried specimens. 



(zooL. CHALL. EXP. — PART Lix. — 1887.) Nnn 21 



