REPORT ON THE TETRACTINELLIDA. 
177 
conditions under which particular forms have been evolved. In the case of Psammastra 
we have seen that somewhat similar cladoxeas are exceptionally met with in Ecionema 
'pyriformis where they are associated with an unusually dense skeleton, consisting of 
massive spicules closely packed together into fibres. In Psammastra murrayi these 
spicules are associated with an exceptional character in the cortex, the abundant presence 
of angular quartz grains, and in both cases the deviation from the average trisene t5rpe 
would thus appear to be due to secondary pressures and tensions, due in the one case 
to the close packing of the spicules, and in the other to the presence of opposing sand 
grains. 
It is possible that the unusual forms produced under these circumstances may 
afterwards become preserved by inheritance, and we may thus explain the presence of 
cladoxeas in the choanosome, where the pressures due to the presence of sand grains do 
not exist. 
In concluding these remarks on the cladoxeas one may mention that they are present 
in greater variety than is described under the heading of spicules, and that the axial fibre 
in the vicinity of the cladal origin frequently gives rise to more numerous branches than 
those which enter the cladi. In some cases, just below the cladal origin, three fibres 
passing off backwards and outwards from the main axis into the substance of the rhab- 
dome have been observed. These accessory branches have no effect whatever on the 
external form of the rhabdome. 
The microstrongyles are scattered through both choanosome and cortex, though they 
are more abundant in the latter, and immediately beneath its investing epithelium they 
occur in a single layer, arranged as closely as possible together without actually touching. 
The asters occur in the inner part of the cortex, but are chiefly distributed through the 
choanosome. 
Appendix to the Euasteosa. 
Family Epipolasidje.^ 
Euastrosa (?) without trisenes, possessing oxeas and one or more forms of aster. The 
oxeas arranged partly in radiating fibres, partly scattered loosely in the choanosome ; in 
the ectosome they lie tangentially. The chamber system (so far as investigated) diplodal. 
Genus Amphius^ n. gen. 
Possessing but one form of microsclere, an amphiaster. Chamber system diplodal. 
^ to lie on the surface, in allusion to the tangential position of the ectosomal oxeas. 
2 a Homeric hero, II., ii. 830. 
(ZOOL. CHALL. EXP. — PART LXIII. 1887). 
Rrr 23 
