with descriptions of some new genera. 405 
dance, as by far the larger number of the fossilized Spongiade 
are composed of this description of tissue, especially in the agates 
of the neighbourhood of Oberstein, and in the green agates, mis- 
called in commerce jaspers, from India. The boulder formation 
of the Yorkshire coast and the shmgle of the Sussex coast also 
produce siliceous pebbles, containing perforated spongeous fibre 
in abundance, and I have similar specimens from various other 
localities. 
In the fossilized state the remains of true Spongia are very 
much less common than those of Verongia, but this difference in 
their comparative numbers in this condition may possibly arise 
in some measure from the greater degree of strength and rigidity 
of the fibre of the latter tribe enabling them to resist the effects 
of accident and decomposition until they were imbedded in the 
silt of the ancient ocean and ultimately fossilized. 
AULISKIA. 
Gen. Char. Skeleton composed of a network of keratose fibre. 
Fibre cylindrical, rigid, continuously fistular, having minute 
cecoid canals radiating from it in every direction. 
The external characters of the fibre of the skeletons of this 
genus are exceedingly like those of Verongia. The colour, the de- 
gree of rigidity and the mode of inosculation agree pr ecisely with 
each other, and it is only when submitted to a careful microsco- 
pical examination by transmitted light with a power of 100 linear 
that the distinguishing character becomes apparent. Under these 
circumstances we find that the fibre is furnished with a conti- 
nuous central canal as in Verongia; but in addition to this organ, 
which permeates every fibre of the skeleton, there are numerous 
minute ceecoid canals, radiating at irregular distances from all 
parts of the great central cavity, as represented by Pl. XIII. 
fig. 1. with a linear power of 100, and by figure 2 with a power 
of 300 linear. 
These secondary canals are given off at nearly right angles to 
the great central one. They are very unequal in length, few, com- 
paratively, reaching to near the external surface of the fibre, and 
none that I have been able to discover perforating it. The greater 
portion of them pursue nearly a straight line from the axis of the 
fibre, a few have a tortuous direction, and a still fewer number 
bifurcate or branch. 
Within the great central canal there were frequently to be 
observed one or two long simple vessels, which might be seen 
traversing the cavity for a considerable distance, as represented 
in Plate XIII. fig. 1. a and 0. 
When there are more than one, as represented at b, they do 
not appear to unite, but run side by side until they reach one of 
