REPORT ON THE MONAXONIDA. 157 



the surface of the sponge ; no secondary skeleton lines appear to be developed, but there are 

 a very great number of loosely scattered megasclera. The fibres themselves are composed 

 each of a central axis of smooth oxeote spicules very abundantly echinated by spined styli. 



Spicules. — (a) Megasclera; (1) smooth, straight, somewhat fusiform, hastately 

 pointed oxea, size about 0'2 by 0"005 mm. (2) Entirely spined, usually slightly curved, 

 sharp pointed styli, which may attain a length of about 0'2 mm., but this is exceptional 

 and they are usually much smaller, measuring about 0*1 by 0*005 mm. Although Carter 

 {loG. cit.) describes two forms of spined stylote spicules, which he distinguishes according 

 to their size, yet, as he himself states, both are echinating, and it appears to us probable 

 that the smaller are simply young forms of the larger, (b) Microsclera ; of one kind, 

 viz., tridentate isochelse, with moderately curved shaft, length up to about 0"02 mm. 



This is a very fine and well characterised species, and as it is as yet very little known 

 (although possibly common in Southern Australia), and as no figures of it are extant, we 

 have thought it advisable to give a somewhat detailed description of it in this place. 



Mr. Carter has very kindly sent us a small piece of his sponge for examination ; we 

 find from it that as regards the form and size of the spicules and the arrangement of the 

 main skeleton, the Challenger sponge agrees sufiiciently closely with that from Port Phillip 

 Heads, but in the latter we have been able to discover no definite dermal reticulation 

 such as we have described above, the dermal skeleton being an irregular reticulation of 

 scattered oxeote and stylote spicules. It must, however, be borne in mind that we have 

 only had a small piece to examine, and that this reticulation may possibly be present in 

 other, and perhaps older, parts of the sponge, hence we have not thought fit to separate 

 the two as distinct species. 



Xoca%.— Station 162, April 2, 1874 ; lat. 39' 10' 30" S., long, 146° 37' 0" E. ; Bass 

 Strait; depth, 38 fathoms ; bottom, sand and shells. One fine specimen. 



Habitat. — Port Phillip Heads, Southern Australia (Carter); Bass Strait, 

 (Challenger). 



Genus Plocamia, Schmidt (Pis. XXIX., XXXI. ). 



1870. Plocamia, Schmidt, Spong. Atlant. Gebiet., p. 62. 



1881. Dirrhopalum, Ridley, Journ. Linn. Soo. Lond. (Zool.), vol. xv. p. 476. 



Sponge of varying form, may be encrusting or erect and branched. Spicules. — (a) 

 Megasclera ; dumb-bell-shaped or sausage-shaped (the characteristic spicules of the 

 genus), and stylote. (6) Microsclera ; isochelae and (at any rate usually) toxa. 



For details regarding the history and characters of this genus the reader is referred 

 to Mr. S. 0. Eidley's paper on the genus Dirrhojxdum {loc. cit., s^ipra). 



The name " Dirrhopalum" was substituted for "Plocamia" on insuflicient grounds, 

 and must be abandoned in favour of the latter. The Challenger only obtained a single 

 specimen of the genus, but this specimen is a very interesting one. 



