PROGRESS OF MICROSCOPICAL SCIENCE. 
141 
M. Fischer has assigned to this species the name of Hydractina cretacea, 
while to Michelin’s species he has given that of Hydractina Michelini. 
To the two examples thus noticed by Michelin and Fischer, Professor 
Allman is enabled to add a third from the Coralline Crag of Suffolk. 
It occurs among some Coralline Crag fossils in the collection of the 
British Museum. It was found encrusting two specimens of Purpura 
lapillus, one from Orford, and the other from Gedgrave, in Suffolk. It 
covers with a continuous crust the shell over which it spreads, and has 
a minutely alveolar structure, with its surface thickly set with short 
blunt spines. The original chitine of the common basal expansion is 
entirely replaced by carbonate of lime. There cannot be the slightest 
doubt of this fossil being a true Hydractinia , and indeed it is impos- 
sible to find any characters which can separate it from the living 
Hydractinia echinata. From the mere fossilized basis, however, which 
is, of course, all that has come down to us, we should not be justified 
in asserting its identity with the living hydroid. 
The Epitheca in Fossil Sponges. — Dr. B. Holl in an article of some 
length explains that in the fossil sponges some portion of the surface, 
especially externally towards the base, is frequently observed to be 
either without jiores, or they are so minute as to be invisible, and the 
sponge then appears as though covered by a more or less smooth or 
slightly-wrinkled membrane, which has been regarded by D’Orbigny, 
De Fromentelle, F. A. Roemer, and others, as analogous to the epi- 
theca of the Zoantlmria ; and the occurrence of this epitheca has been 
held to be an additional evidence of the stony nature of the sponge 
skeleton. When examined microscopically, by means of thin sections, 
however, it appears that this epitheca is due to the filling up of the 
interstices of the superficial parts of the sponge, which, in the situa- 
tions in which it exists, is finer and more condensed than elsewhere. 
This greater density at the surface may be seen in many recent sponges, 
the superficial portions of the tissues being closer and finer than that 
of the interior, which was formed during an earlier and more active 
period in the growth of the sponge. But either from having arrived 
at maturity, or at a period when the growth was temporarily arrested — 
for in some sponges the growth is intermittent — or, as appears some- 
times to be the case, from some local cause, the tissue at the surface 
assumes a closer arrangement. Thus in the common Halichondria 
panicea, the surface over greater or lesser portions frequently pre- 
sents a condensed appearance with scarcely any visible interspaces, 
the outer superficial portion being made up of a densely-matted layer 
of spicula placed for the most part parallel to the surface ; and the 
same is true of many fibrous sponges, as shown by Dr. Bowerbank. In 
some of the fossil sponges a similar modification of the tissue at the 
surface appears to have obtained, especially in certain cup-shaped and 
cylindrical sponges, and in the calcareous fossils in which this has 
been the case, the interstices, from their extreme minuteness, are more 
or less filled with carbonate of lime. Thus a species of Cupulospongia, 
common in the gravel-pits of Farringdon, frequently presents- on its 
interior a smooth surface, described by the late Daniel Sharp as a 
membrane. But if a number of individuals of this species be ex- 
