282 
THE VOYAGE OF H.M.S. CHALLENGER. 
cladus 0'045 mm,, tetracladus 0‘02 mm.; maximum diameter of the calthrops, measured 
from the extremities of the most remote rays, 0‘324 mm. 
Habitat, — Seychelle Islands, in deep water. 
Remarks. — The description of the calthrops is taken from a slide kindly presented me 
by Mr. Carter, who found it associated with debris at the root of a specimen of JEuplectella 
cucumer from the Seychelles. The spicule is unique in form, and brings to mind the 
tetracladine desmas of the Lithistida. The resemblance extends into details, the actinomes 
being thickened ridge-like on their confluent sides, so as to produce a triangular depression 
at each point of union of the actinomes taken in threes. A precisely similar character 
will be found described in Theonella sivinhoei, pp. 285, 290. 
The chief distinction between this spicule and a Lithistid desma is to be found in the 
axial fibre, which in the spicule extends throughout the cladi up to the extremity of the 
tetracladus, while in the Lithistid desma it never extends past the protocladus ; another 
probably equally important difference is presented by the cladi, which are more or less 
cylindrical or conical according to their position, but never expanded into laminae, and 
rigidly straight, never crooked. The difference in size is also worthy of remark. 
[ Corticium parasiticum, Carter. 
Corticimn parasiticum, Carter, Ann. and Mag. Nat. Hist., ser. 4, vol. xviii. p. 229, pi. xvi. 
fig. 52, 1876. 
This species evidently belongs to the Monaxonida. 
Family III. Thrombida:. 
Thrombus abyssi (Carter.) 
Coi'ticium abyssi. Carter, Ann. and Mag. Nat. Hist., ser. 4, vol. xii. pp. 18-22, pi. i. figs. 1-9, 
15, 1873. 
Sponge . — Reptant, amorphous ; surface smooth, slippery ; oscules numerous, small, 
of different sizes, congregated now and then irregularly ; pores simple, not in sieves, 
uniformly scattered ; mesoderm similar to that of Thrombus challengeri. 
Spicules . — 1. Trichotrisene, rhabdome straight, ornamented with conical erect 
spines, except between the actinal and cladal centres ; esactine conical, oxeate ; ecactine 
cylindrical, strongylate ; cladome arising from the middle of the ecactine, cladi tricho- 
tomate, protocladi smooth, deuterocladi spined like the rhabdome. Rhabdome 0‘0583 
mm. long, chord of the cladome 0‘0542 mm. 
2. Amphiaster, axis straight, slender, bearing four recurved spines at each end ; 
0’005 mm. long. 
