48 GUIDE TO THE EOSSIL INVEETEBKATE ANIMALS. 
Q-allery X. 
Table-case 
9 . 
Table-case 
9 . 
Wall-ease 
6c. 
Table-cases 
9 & 10 . 
Wall-case 
6 . 
Between 
Wall-cases 
7 & 8 . 
The Order Gymnoblastea includes both solitary -and 
colonial forms ; among the latter the family Podocorynidae 
contains genera whose roots form nn incrustation, somewhat 
similar to that above described, but without tubes for the 
polyps ; these generally coat gastropod shells. Fossils 
referred to llydr actinia, one of these genera, are found in 
the Pliocene and Upper Cretaceous rocks of England and 
elsewhere. 
There are a number of fossils that present a general 
resemblance to the massive Hydrocorallines or the encrust- 
ing Hydr actinia, and, in the absence of convincing evidence 
for or against, they are usually placed with the Hydrozoa. 
Such are the thick lamellar masses of Mlipsactmia and 
Sphacractinia from Upper Jurassic ro(hs, the spheroidal 
Parkcria from the Cambridge Greensanil and its ally 
Stoliczkaria, and some thin incrustations from the Chalk. JiT] 
Apparently allied to some of these forms are the 
Stromatopores, of which a large series is shown in polished 
slabs of Devonian marble, and a smaller series from the 
Silurian, both of Britain. There are also .several foreign speci- 
mens. These fossils usually began by incrusting some solid 
body, such as a shell, and often grew to enormous size, form- 
ing huge banks and reefs. They are composed of calcareous 
lamime, separated by distinct intervals across which run 
vertical or radial pillars. Both pillars and laminae are 
usually traversed by minute irregular canals. The whole 
mass is in some genera, e.g. k)tromatop>ora, perforated by 
larger tubes, divided by horizontal platforms or tabulae ; in 
these probably lived the principal })olyps. The chief work 
on these obscure fossils is a monograph by H. A. Nicholson, 
published by the Palaeontograpbical Society (1880-1892). 
Class ANTHOZOA. 
Among living forms of this Class, a larger proportion 
have a skeleton capable of preservation in tbe fossil state. 
Eecent Anthozoa are divided into two Sub-Classes: 1. 
Alcyonaria, with 8 mesenteries and 8 tentacles, which bear 
short branches. II. Zoantharia, with C, 12, 24, or a larger 
number of mesenteries and an equivalent number of tentacles, 
wdiich are simple. 
The ALCYONARIA, with few doubtful e.xceptions, live 
in colonies, and the polyps are held together by a jelly 
(mesogloea). Cells wander from the ectoderm into this jelly, 
