372 H. K. A. Zittel on Fossil Calcispongice. 
living Leucones forms with an absolutely identical arrange¬ 
ment of the water-canals. In this respect, therefore, the fossil 
calcareous sponges most approach the Leucones ; but their 
union with these is decisively opposed by the extremely pecu¬ 
liar arrangement of the spicules in fibre-trains. 
In general there is a close agreement in the distribution and 
arrangement of the skeletal elements within the different 
families of the Calcispongise. Thus, for example, in the 
Ascones the triradiates lie almost all in a single layer and are 
completely imbedded in the syncytium ; of the quadriradiates, 
on the contrary, the three facial limbs are placed entirely in 
the plane of the dermal lamella, whilst the fourth or apical 
ray projects freely into the stomachal cavity. The bacillar 
spicules also are originally entirely enclosed in the syncytium ; 
but subsequentlj" they usually protrude more or less, and form 
tufts, circlets, and bristles. 
The spicules in the Sycones are exceedingly regularly dis¬ 
tributed. The composition of the wall of radial tubes gives 
rise to peculiar conditions of differentiation in the skeleton and 
to a definite radial arrangement of the individual parts. We 
consequently always distinguish the true skeleton of the wall 
and radial tubes from the skeleton of the gastral and dermal 
faces. The former usually consists only of triradiates, very 
rarely of bacillar spicules or quadriradiates ; in general the 
triradiates form several parallel layers, and are always so ar¬ 
ranged that the sagittal ray turns outwards, while the two 
lateral (usually short) arms diverge obliquely inwards nearly 
in the same plane. In the dermal and gastral skeletons also 
the spicules are remarkable for their regular arrangement. 
The skeletal elements of the Leucones are quite otherwise 
arranged. In them also the outer surface and that of the 
stomachal cavity exhibit a constitution different from that of 
the true wall; so that we can likewise distinguish a dermal, a 
parenchymal, and a gastral skeleton. In the interior of the 
thick wall the parenchymal skeleton is composed of a mass 
of calcareous spicules of different forms and sizes, as it were 
thrown together without any definite arrangement. The tri¬ 
radiates generally predominate; and with them a few quadri¬ 
radiates and bacillar spicules are intermixed. The surface of 
the Leucones is either smooth or spiny. The smooth dermal 
skeleton is produced by the spicules being pressed close to¬ 
gether, and rather more regularly arranged than in the interior 
of the wall. The spiny dermal skeleton is formed by large 
bacillar spicules which project beyond the surface with their 
distal part. The smooth or spinose skeleton of the gastral 
surface is produced in the same way as the dermal skeleton. 
