274 



HARDWICKE'S SCIENCE-GOSSIP. 



of the so-called corals, as the Millepores, he has proved 

 to be not corals at all, but that they actually belong to 

 the Hydrozoa. He has shown that there is a peculiar 

 division of labour in the polyps of modern Millepores, 

 some of the zoophytes catching the food and others 

 digesting it, after they have received it from the 

 catchers. This is the case in Stylaster, where the food- 

 catching zoophytes very much resemble the tentacles 

 arranged round the mouth of the common sea- 

 anemone. 



Fig. 211. — Sti-omatopora concentr'ica. (Upper Silurian and 

 Uevonian formation), now believed to be a calcareous sponge ; 

 a. surface of fossil ; b, vertical section ; c, portion of Stroma- 

 tofora concentrica magnified. 



The abundant recent coral Heliopora caruha (whose 

 specific name comes from the bright blue colour of 

 the stony structure, which is usually white) is an 

 Alcyonarian, more nearly allied to some of the sea- 

 fans than to true corals. It is plentiful in equatorial 

 seas, and especially off the Bermudas. It has not in- 

 distinct traces of septa. The Heliolites so abundant 

 in the Silurian and Devonian limestones, do not differ 

 in any important particular from the living Heliopora, 

 and like it they no doubt belonged to the Alcyonaria. 



The division called Rugosa, on the other hand, is 

 distinguished by well-marked septa, radiating from 

 the coral walls towards the centre, in the pretty star- 

 shaped fashion which caused Cuvier to group these 

 creatures along with other similarly star-rayed in 

 their shapes, into the sub-kingdom Radiata, now no 

 longer accepted by naturalists. In this radiated 

 structure, therefore, the Rugose corals resemble the 

 Aporose corals. But whereas the Tabulate and 

 Rugose corals (with few exceptions) are limited to 

 Palaeozoic rocks, the Aporose corals are peculiar to 

 those formed since then. Again, the septa, or 

 radiating ridges, of the Rugose corals are always in 

 multiples olfour; whilst those of Aporose corals are 

 in multiples of six. Besides this means of distin- 

 guishing the Aporose corals from any of the others, 

 the fact that they never have tabulae — that is, are 

 never divided into horizontal layers — is another im- 

 portant distinction. When the tabulate corals have 

 faint traces of septa, we can see they are also in 

 multiples of four, and they thus show their structural 

 relationship to the Rugosa. Dr. Sorby has shown that 

 the tabulate corals are built up of Calcite, whilst the 

 Neozoic and modern corals are formed of that limy 

 structure known as Arragonite. It may be that the 

 Rugosa are descended from the Tabulata, which would 

 at once make clear why the tabulate corals appear 

 in such numbers of species and in individuals in the 

 Silurian and Devonian seas. In the Carboniferous 

 rocks the most numerous corals are the Rugose kind, 

 in which the radiated structure is very plainly visible, 

 as in Lithostrotion junceum, &c, of which we give 

 illustrations of the transverse appearance they present 

 when cut and polished. For some of our illustrations 

 we are indebted to Mr. James Thomson, F.G.S., of 

 Glasgow — one of the most enthusiastic and diligent 

 students of Palaeozoic fossil corals in Europe. The 

 numerously represented fossils called Stromatopora 

 (fig. 21 1) — so abundant in our Silurian and Devonian 

 limestones (in the latter, about Plymouth and Torquay 

 so plentiful that rock-masses are composed of them 

 alone) — are now believed to be Calcareous sponges, 

 or sponges whose abundant limy spicules amal- 

 gamated into the concentric rings which are charac- 

 teristic of their structure. The student may study 

 this structure in any polished mantel-piece formed of 

 Devonian marble from the quarries of Newton- Abbot 

 and the neighbourhood. By far the prettiest of the 

 Palaeozoic fossil corals are those belonging to the 

 Rugosa, such as Strombodes, Cyathophyllum (per- 

 haps the most plentiful of them all), Cyathaxinia 

 (a simple coral), Lithostrotion, Lithodendron, &c. 

 Perhaps the true simple coral which may be regarded 

 as the simplest in structure is Amplexus, and there is 

 reason for believing that many more elaborate fossil 

 corals pass through a kind of Amplexus stage. 



We shall have to devote separate articles to generic 

 descriptions of fossil corals, and localities where they 

 are most abundant. 



