1875.) SILICEO-FIBROUS SPONGES, 563 
structure of an irregular form, of an average diameter of half an inch. 
It is constructed of two, and in some parts of three, layers of rete, the 
intermediate laver, when present, having its fibres more slender and 
more irregularly disposed than the two external ones. The fibres of 
the outer structures are remarkably large and strong; an average- 
sized one measured ;4-~ in. in diameter; and the rete is more than 
usually close and compact; in many cases the areas do not 
exceed in breadth the diameter of the surrounding fibres. The 
form of the rete is mostly either square or oblong; and its strength 
is greatly increased by the interior angles being replaced by curves, 
so that the areas are to a great extent either circular or oval. The 
external layer of tissue is sparingly spinous, and is also furnished 
with short and slender aculei, and abundautly with rectangulate 
sexradiate defensive organs, based most frequently on the sides of 
the fibres and projected thence into the areas of the network ; and 
the aculei are apparently the nascent state of these organs. 
There are strong appearances of the specimen having been part 
of a cup-shaped sponge; at one portion of it the skeleton-rete is 
closely and irregularly as it were crushed together; from this 
part the primary skeleton-fibres radiate in a fan-shaped mode, the 
secondary ones assume the state of a series of concentric curves, and 
the reticulation increases in regularity of structure as it approaches 
what has evidently been the distal-portion of the cup-shaped struc- 
ture; and here it is that we find the dermal membrane and its cha- 
racteristic spicula in the finest state of preservation. 
The dermal membrane is extremely pellucid, and would scarcely 
be visible if it were not for the numerous retentive spicula adhereut 
to its surface. 
The bihamate retentive spicula are numerous, uniform in size, and 
very slender; their curves are about three fourths of a circle; and 
they are equably dispersed over the surface of the membrane. They do 
not exceed =4+,inchin length. The minute bidentate anchorate ones 
are very few in number; one of the largest of them measured S006 
inch in length: they vary in their mode of development to some 
extent; and occasionally there is a third, small tooth, more or less 
produced, between the two large lateral ones. The attenuato-subspi- 
nulate internal defensive spicula are also very numerous; they 
vary somewhat in size, and are entirely incipiently spinous; one of 
the largest measured ;}, inch in length; they are based on the 
inner surface of the dermal membrane, and are projected inwards at 
various angles, while others are seated on the sides of the dermal 
skeleton-rete, and are projected into the areas at various angles. 
The whole three forms abound, not only on those parts of the mem- 
brane covering the areas of the reticulations of the fibres, but they 
also occur on the parts attached to the skeleton-fibres, so that no 
part of the dermal membrane is left unprotected. 
The central canals of the skeleton-fibres are small ; they vary to 
a slight extent in some parts of the rete, and in some of the largest 
of the fibres they are entirely obsolete. 
The rectangulate sexradiate organs are not very numerous near 
