COELENTEEA — GEAPTOLITES. 
47 
reduced, e.g. Loganogra^tus, Tctragraptus, until there arose a 
form with only two saw-blades to a single handle, e.g. Didy- 
mograptus (Fig. 19). If these hnng from sea- weed the polyps 
would be mouth downwards, a position obviously ill adapted 
for securing food-particles showered from the floating weed. 
This was remedied in two different ways. In one way, the 
two blades, instead of hanging down like a pair of tongs, 
gradually opened until at last they were directed upwards. 
In the other way, the polyps of the first two thecae seem to 
have stretched upwards and so to have bent their thecae in 
that direction ; the rest followed suit, and grew upwards 
along the thread from which the sicula hung, e.g. Diplo- 
graptus. Thus was formed quite a different type of graptolite. 
Further developments took place in these latter : the colony 
acquired a median supporting rod or virgula; this ended 
often in a disc, which, it is supposed, was hollow, and served 
as a float. The colonies were often compound, and many vir- 
gulae with their thecae were attached to a single disc. If the 
supposition that such forms took to a free-floating existence 
be correct, we shall understand why succeeding forms, pre- 
sumably their descendants, should have had their periderm 
formed of a meshwork of fine strands, e.g. Retiolites : this is a 
well-known way of obtaining lightness without loss of strength. 
Such are the main lines of graptolite evolution up to 
their abrupt end at the close of the Silurian Epoch. But 
there were many subsidiary lines ; and all these, combined 
with the wide distribution of each successive form owing to 
its floating life, have rendered the graptolites of enormous 
value to the geologist in determining the succession of layers 
in great thicknesses of rock, and in tracing those layers over 
a large extent of country, even when much disturbed by 
later earth-movements. A monograph of the British species 
is in course of publication by the Palaeontographical Society. 
Fossils tliat can with certainty be referred to the 
Hydrozoa are very few, and are not older than Cretaceous. 
They are confined to forms in which the ectoderm secretes 
outside the polyp or zooid a number of small calcareous 
rods ; tliese grow together into a solid mass, leaving tubes in 
which are the polyps. Most of these forms are generally 
called Hydrocorallines, lately divided into Stylasterina and 
Milleporina, of which only the latter are found fossil ; and, 
indeed, Millcpora itself is not known in rocks older than 
some Pleistocene raised beaches. The Eocene fossil Axopora 
is but a doubtful ally. 
Gallery X. 
Table-case 
9 . 
