48 GUIDE TO THE FOSSIL INVEETEBEATE ANIMALS. 



Gallery X. The Order Gymnoblastea includes both solitary and 

 colonial forms ; among the latter the family Podocorynidae 

 contains genera whose roots form an incrustation, somewhat 

 similar to that above described, but without tubes for the 

 polyps ; these generally coat gastropod shells. Fossils 



Table-case referred to Hydradinia, one of these genera, are found in 

 ^' the Pliocene and Upper Cretaceous rocks of England and 

 elsewhere. 



Table-case There are a number of fossils that present a general 

 resemblance to the massive Hydrocorallines or the encrust- 

 6c. mg Mydrachma, and, m the absence oi convincing evidence 

 for or against, they are usually placed with the Hydrozoa. 

 Such are the thick lamellar masses of Mlipsactinicr and 

 Sphaeractinia from Upper Jurassic rocks, the spheroidal 

 Parkeria from the Cambridge Greensand and its ally 

 Stoliczkaria, and some thin incrustations from the Chalk. 

 Apparently allied to some of these forms are the 



Table-eases Stromatopores, of which a large series is shown in polished 



WaU case ^^^^^ Devonian marble, and a smaller series from the 

 6. Silurian, both of Britain. There are also several foreign speci- 

 Between mens. These fossils usually began by incrusting some solid 



^7^&^8^^^ body, such as a shell, and often grew to enormous size, form- 

 ing huge banks and reefs. They are composed of calcareous 

 laminse, 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. Stromatopora, perforated by 

 larger tubes, divided by horizontal platforms or tabulae ; in 

 these probably lived the principal polyps. The chief work 

 on these obscure fossils is a monograph by H. A. Mcholson, 

 published by the Palaeontographical Society (1886-1892). 



Class ANTHOZOA. 



Among living forms of this Class, a larger proportion 

 have a skeleton capable of preservation in the fossil state. 

 Ptecent Anthozoa are divided into two Sub-Classes : I. 

 Alcyonaria, with 8 mesenteries and 8 tentacles, which bear 

 short branches. II. Zoantharia, with 6, 12, 24, or a larger 

 number of mesenteries and an equivalent number of tentacles, 

 which are simple. 



The ALCYONARIA, with few doubtful exceptions, live 

 in colonies, and the polyps are held together by a jelly 

 (mesoglcea). Cells wander from the ectoderm into this jelly, 



