SILURIAN FAUNA. 143 



foims of life — that the representatives of each class of animals were 

 first ushered in in their simplest or most embryonic forms, and that 

 not until these had attained a considerable development was there a 

 noticeable appearance of the more highly constituted forms. It is 

 a significant (even if not a very remarkable) fact that, prior to the 

 first introduction of this lowest class of the Vertebrata, all the 

 larger divisions of the Invertebrata, as now recognised by natu- 

 ralists, had already come into existence. Of these, the diversity 

 of form in the Silurian deposits, no less than the numerical de- 

 velopment, is very great, and equally so in almost all the classes 

 represented. 



The most marked feature of the Silurian invertebrate fauna, as 

 contrasted with the Cambrian, is furnished by the corals, which, 

 barring a few forms doubtfully belonging to the Cambrian of 

 Sweden, have here their earliest representatives. These primitive 

 types of the Actinozoa, as well as nearly all others of the Paleozoic 

 series of deposits, have generally been recognised by naturalists to 

 constitute two well-defined groups, the Tabulata (Favosites, Haly- 

 sites, Heliolites, Alveolites, &c.) and the Rugosa, or cup-corals (Cy- 

 athophyllum, Streptelasma, Omphyma, Zaphrentis, &c.), in both of 

 which the calyces are divided up into superimposed chambers by 

 transverse plates or tabulee — the former with very rudimentary septa, 

 the latter with the septa well developed, and the outer calicular 

 wall greatly thickened. In the majority of these cup-corals the 

 septa are disposed in multiples of four (Tetracoralla), whereas in 

 nearly all recent Madreporaria this disposition is effected in multi- 

 ples of six (Hexacoralla). Our recently acquired knowledge of the 

 deep-sea fauna, and a more intimate acquaintance with the anatomy 

 of some of the more aberrant species of coral, tend to show that 

 the supposed sharp delimitation of the Paleozoic actinozoan fauna 

 does not in reality exist. The tabulate corals, for example, whose 

 final extinction with the Paleozoic era has generally been insisted 

 upon as one of the most decisive of geological landmarks, would 

 seem to hold a number of forms more or less closely related to 

 types living in the modern seas, which in themselves combine most 

 diverse features in their organisation. The genera Halysites and 

 Syringopora appear to be not distantly removed from the recent 

 organ-pipe (Tubipora); Favosites is placed among the -Poritidse; 

 and Heliolites not impossibly represents an ancestral form of the 



