FOSSILS— THEIR NATURE AND INTERPRETATION. 9 1 



of six. True Hexacoralla have not been discovered below 

 the Ordovician, or second geological period. 



The diagram Fig. 7 illustrates the fundamental elements 

 of a coral (Hexacoralla). 



hs ,ss mp \hs .«s 



1 



Fig. 7. 



Fig. 9. 



Fig. 7. — Diagram of the structure of a coral : ap = exotheca ; /ts = mesentery ; yp — basal plate ; 

 ss= septa ; parts in white = calcareous skeleton ; shaded = ectoderm ; black = mesogloea ; 

 dotted = endoderm. (After IMcMurrich.) 



Fig. 8. — Diagram of an end view of a single corallum of Favosites, showing the rudimentary sep- 

 ta ss, the dotted lines indicating the probable arrangement of the mesentery and the position 

 of the mouth opening o. 



Fig. 9. — Diagram of two chambers of a corallum of Favosites with the perforated walls, and the 

 transverse dissepiment or tabulae, ii', separating the chambers. 



The calcareous tube or support of each animal (polyp) is 

 the corallum, the wall iaf) is the thcca, the longitudinal parti- 

 tions {si) are the septa. The septa radiate toward the centre 

 and are in multiples of 4 in the forms called Tetracoralla and 

 in multiples of 6 in the forms called Hexacoralla. 



The characteristics of the Hexacoralla cup are expressed 

 in the specimen before us, the Favosites niagarensis ; the 

 septa are, however, in only rudimentary condition, appearing 

 in the fossil forms only as faint ridges or rows of spinous pro- 

 jections on the inside of the tubes, as in the diagram Fig. 8. 

 The transverse partitions (see Fig. 6, h) are basal plates, con- 

 structed as the corallum grows upward for the animal to rest 

 upon, and are called tabulae or dissepiments. The Favosites 

 are characterized by the prominent development of tabulae, 

 from which character the corals of this type are called 

 tabulata. The specimen presents another character (see Fig. 



