with descriptions of some new genera. 405 



dance, as by far the larger number of the fossilized Spongiada 

 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 shingle 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 efiects 

 of accident and decomposition until they were imbedded in the 

 silt of the ancient ocean and ultimately fossilized. 



AtJLISKIA. 



Gen. Char. Skeleton composed of a network of keratose fibre. 

 Fibre cylindrical, rigid, continuously fistular, having minute 

 ca3coid 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 precisely 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 caecoid canals, radiating at irregular distances from all 

 parts of the great central cavity, as represented by PI. XIII. 

 fig. 1. with a linear power of 100, and by figure 2 with a power 

 of 300 linear. 



These secondary canals are given ofi^ 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 h. 



When there are more than one, as represented at i, they do 

 not appear to unite, but run side by side until they reach one of 



