84 GEOLOGICAL BIOLOGY. 



bite is characteristic of the Paleozoic, and the Ichthyosaurus 

 is characteristic of Mesozoic time, as truly as man is charac- 

 teristic of recent time. 



Stony Corals: the Zoantharia. In order to emphasize and 

 illustrate this law of the intimate connection between organic 

 form and time, the statistics regarding the great order of the 

 stony corals (the Zoantharia) may be chosen. 



For the convenience of those who may have no special acquaintance with 

 the scientific nomenclature of systematic Zoology, a few facts regarding the prin- 

 ciples of classification and nomenclature are here offered. The classification of 

 animals is based primarily upon differences in form, structure, and function. 

 On this basis zoologists have classified animals under nine chief divisions, called, 

 I, Protozoa ; 2, Ccelenterata ; 3, Echinodermata; 4, Vermes; 5, Arthropoda; 

 6, Molluscoidea ; 7, Mollusca ; 8, Tunicata ; 9, Vertebrata. (Claus.) Each of 

 these divisions is called a Branch (Phylum or Subkingdom) of the Animal King- 

 dom, and each is characterized by a distinct type of organic structure. 



Under each of these chief divisions the animals are associated by their 

 greater degrees of likeness, and are separated by their lesser differences, into 

 subdivisions, called respectively, from higher to subsidiary rank, Classes, Orders, 

 Families, Genera, and Species. The Ccelenterata are thus at present known 

 under four Classes, viz., Spongia, Anthozoa, Hydrozoa, Ctenophora. 



The class Anthozoa (coral animals) is subdivided into two orders, Alcyonaria 

 and Zoantharia. The order Zoantharia is subdivided into three suborders: The 

 Antipatharia^ the Actinaria, and the Madreporaria. The first two of these 

 suborders develop no hard parts that have been recognized in a fossil state, and 

 therefore we cannot speak of their historical relations. The Madreporaria are 

 the polyps which secrete stony corals, and of their calcareous skeletons great 

 numbers have been found in the rocks; many massive beds of limestone consist- 

 ing mainly of them or their fragments. 



The Madreporaria, or stony corals, have been classified in two groups of 

 families, the most characteristic feature separating them being the arrangement 

 of the septae in one of them in fours or multiples of four, Tetracoralla, and in 

 sixes or multiples of six in the other group, Hexacoralla. 



Numbers of Genera of the Zoantharia recorded for each Era. 

 There are several thousand species of stony corals described, 

 but for the present purpose it is sufficient to note that there 

 are 448 genera of Zoantharia already described and recog- 

 nized. (Zittel.) That is, there are 448 different combina- 

 tions of form of the stony corals, which are sufficiently sharply 

 defined and constant in their character to be classed under 

 distinct genera. If we only note the numerical relation of 

 these genera to the successive geological periods of time, the 

 law above referred to becomes at once apparent. In the 

 Lower Silurian 4 genera are reported by Zittel ; 5 genera have 



