74 
Dr. Wallich, on the Polycystina, 
surfaces of the spines being strengthened by longitudinal ribs, 
which are again sometimes flanged at their free margins. 
Some idea of their form may be gained on looking at the 
section of an ordinary railway plate, 
A peculiarity observable in the nucleus of this family con- 
sists in its being moulded, as it were, to and around the 
siliceous axis of the spines, whilst the nuclear capsule sends 
off processes which closely invest the spines to their extremi- 
ties."^ This character, seemingly so abnormal, results from 
the rudiments of the spines being developed within the sarco- 
blast after the nucleus has become invested with its capsule, 
and each spine, as it extends outwards, thus pushing the cap- 
sule before it. I may observe that I possess the clearest 
evidence of this from the progressive stages in which the 
sarcoblasts of the Acanthometrina are constantly met with in 
tropical seas. 
The spines of the family furnish the type of a portion of 
the siliceous structure both of the Polycystina and the Rho- 
zopods of the second order, which demands that they should 
receive a name indicative of their origin and distinct cha- 
racter. I accordingly propose the term acanthostype,-\ Their 
value in furnishing some of the most useful characters for 
classifying will be explained hereafter. But there is reason 
to believe that the acanthostype of the Acanthometrina gene- 
rally is not so purely siliceous as that of the other families 
into the composition of whose skeletons siliceous material 
enters, inasmuch as its index of refraction is not the same ; 
and, besides occasionally assuming a delicate roseate tint, it 
may be made to yield to solvents much more readily than 
any siliceous spicules with which we are acquainted. This, 
perhaps, affords an explanation of the very curious fact that 
in all deep-sea deposits and fossil earths I have never disco- 
vered a trace of an Acanthometra ; nor am I aware of the 
skeleton of one having been met with by others in thpse 
deposits, or in the flints. The Acanthometrina are essentially 
free-floating, and, without exception, marine organisms. 
Of the Thalassicollid(B it is not requisite that I should 
speak more in detail at present, except in so far as these 
characters demand comparison with those of the Rhizopods 
* I have been unable to satisfy myself as to whether, in the living and 
normal state, the capsule is ultimately ruptured at the apices of the spines. 
In some examples they certainly have been so ; but the extraordinary sen- 
sitiveness of Acanthometrina to the change of circumstances resulting from 
temporary imprisonment in the toviring-net, and examination subsequently, 
renders it, in my opinion, probable that the rupture is an abnormal con- 
dition. 
f AkuvQ, and (ttv-ttoq, a stem or stock. 
