168 BRITISH PALEOZOIC SPONGES. 



rays, at a short distance from the central axis, divides into three secondary pointed 

 tapering rays. The diameter of this form is - 4 mm., and the thickness of the rays 

 near the centre is "04 mm. These spicules are very unfavorably preserved ; they 

 are not uncommon in a boulder of chert from the Drift at York, associated with 

 spicules of Beniera and other forms like those in the chert-beds at Richmond ; it is, 

 therefore, probable that it may originally have been derived from this neighbour- 

 hood. A flesh-spicule of a nearly similar form, but of very much smaller proportions, 

 has been figured by Dr. Bowerbank from Euplectella asjoergillum, Owen. 1 



Sub-Order. — HeteractinellievE. 

 Genus. — Tholiasterella, 2 Hinde, gen. nov. 



Syn. — Holasterella (in part), Hinde ; ? Hyalonema (in part), Garter. 



Form of Sponge unknown ; the skeleton consists of spicules, which, as suggested 

 by Mr. Carter, bear a general resemblance to the handle and ribs of an umbrella. 

 The handle or vertical ray of the spicule supports on its summit a variable number 

 of rays which radiate from it in a generally horizontal direction. A central disc of 

 variable proportions is formed by the union of the bases of the horizontal rays and 

 the upper surface of this, and of the rays, may be either smooth or covered with 

 tubercles or blunted vertical spines. In some cases spicules of an irregular form 

 are present, in addition to the normal umbrella-spicules. 



The spicules of the body of the Sponge appear to have been aggregated 

 together without definite arrangement ; they seem to have been mostly free from 

 each other, and merely held in position by the interlacing of their rays ; but in 

 some cases the rays appear to have been partially cemented together. The outer 

 surface or dermal layer of the Sponge consisted of a framework with irregular 

 interspaces, formed by the intervening and partial fusion of the horizontal rays of 

 larger and smaller " umbrella " spicules, whilst the shafts of these spicules pene- 

 trated into the interior of the Sponge. 



The spicules on which this genus is based were recognised both by Messrs. 

 J. and J. Young and by Mr. Carter, and in the ' Cat. Foss. Sponges ' I described 

 them as a species of Holasterella. 



Since then Mr. James Bennie, Mr. John Smith, and also Mr. John Young, 

 have supplied me not only with many fresh examples of detached spicules, but 

 with fragments of the skeletal-structure in which the spicules are in their original 



1 * Mon. of the Brit. Spongiada;,' vol. i, p. 258, pi. viii, fig. 1S9. 



2 doXla, a hat to keep the sun ofF, a parasol ; uariip, a star, dimin. 



