Dr. G. J. Hinde on Fossil Calcispongic. 191 
these examples have suffered a similar change to that which 
has happened with the Cambridge-Greensand specimens ; 
namely, the interior portion of the fibre remains calcareous 
and can be removed by acid, whilst the exterior, in immediate 
contact with the flinty matrix, has become silicified. This fact 
very clearly shows the secondary origin of the siliceous enve- 
lope of the fibres of Pharetrospongia. 
- It seems to me therefore that the originally calcareous struc- 
ture of the Pharetrones is strongly supported by the present 
condition of these fossils and the contrast between them and 
the remains of siliceous hexactinellid and lithistid sponges 
which are present in the Lower and Upper Greensand and the 
Chalk. 
The new facts which I have to bring forward respecting 
the spicular structure of the Pharetrones mainly rest upon 
the discovery that, in certain sponges of this group from the 
Upper Greensand of Warminster, the fibre is in such a loose 
semi-friable condition that the spicules composing it may be 
obtained, in some instances quite, in others partially, free 
from the matrix, thus enabling their true forms and propor- 
tions to be ascertained with a degree of accuracy which it has 
been impossible to arrive at by a study of thin microscopic 
sections. ‘These Warminster sponges not only presented this 
peculiarity of the fibre, but they also exhibited on the outer 
or dermal surface a layer of spicules differing in form and size 
from those composing the inner fibre, and sufficiently large in 
some instances to be visible with a strong simple lens. This 
discovery led me to make a special search for these surface- 
spicules, and resulted in proving their occurrence on the ex- 
terior of several different genera of Pharetrones both from 
Cretaceous and Jurassic strata. The significance of this dis- 
covery will be understood from the fact that in their character 
and position these surface-spicules, as will be hereafter shown, 
strikingly resemble those of existing Calcisponges. . 
The Warminster sponges above referred to belong to two 
genera (Verticillites, Det., and Corynella, Zittel) ; but they 
appear to me to constitute new species. In common with the 
other calcareous sponges from these beds, these retain their 
outer form well preserved and mostly free from the matrix, 
which consists of a somewhat coarse-grained quartzitic sand 
with green particles, probably of glauconite. The fibres of 
the skeleton have a dull white aspect, alike in the interior and 
on the exterior surface; and, as already mentioned, they are 
in places partially disintegrated, and break up into a soft 
powdery material, in which many of the spicules remain, 
though for the most part fragmentary. Subjoined are de- 
