DISTRIBUTION OF COEALS. 247 



corals is likewise exemplified in the case of the reef-builders ; but 

 here the absolute limitations to existence that are set by the physi- 

 cal aspects of the environs render such a condition in no way sur- 

 prising ; indeed, it is just what might have been expected. Only 

 a very limited number of the existing generic types date from the 

 Cretaceous period (Porites, Diploria, Maeandrina, Goniastraea) and a 

 still smaller number from the Jurassic (Favia, Heliastraea, Cladocora), 

 although not unlikely all, or very nearly all, of them belong to some 

 part or other of the Tertiary period. On the other hand, the num- 

 ber of generic types that range through the whole series of the 

 Mesozoic deposits (Isastraea, Thamnastreea, Rhabdophyllia, Thecos- 

 milia, Cladophyllia, Latimaeandra, Stylina) is considerable, and 

 some of these, as Thamnastrsea, Thecosmilia, and Rhabdophyllia, 

 also pass over into the Tertiaries. 



In their geographical distribution the modern reef -builders are 

 confined to a zone extending on either side of the Equator whose 

 outer limits are bounded by the isocryme of 68 Fahr., or the line 

 which marks a lowest average temperature of 68 for all months of 

 the year. No species, apparently, can endure a lower temperature, 

 while most of them require for their greatest development a tem- 

 perature considerably more elevated. Professor Dana has divided 

 the coral-reef seas into two primary sections, the torrid and the 

 sub-torrid, the former of which, whose delimitation is fixed by the 

 isocrymes of 74, is included in principal part between the twentieth 

 or twenty-third parallels of north and south latitude, although 

 reaching in the Red Sea and the Gulf of Mexico considerably 

 beyond the normal limit. The greatest profusion and wealth of 

 coral growth is exemplified in this region, and particularly in the 

 waters of the Central Pacific (Fiji islands, &c.). The astraeas, 

 maeandrinas, porites, and Pocilloporae attain here their fullest per- 

 fection, and with them are associated large beds or masses of ma- 

 drepores, Pavoniae, Fungiae, and tubipores, and hosts of other forms 

 of the most diverse outline and brilliancy of colouring. A deficiency 

 in the variety of species and genera becomes apparent as we pro- 

 ceed eastward. Much the same types as occur in the Pacific are 

 represented in the coral islands of the Red Sea and the Indian 

 Ocean, and in the East Indies ; and in the limited fauna which is 

 developed along the western coast of America, between Guayaquil 

 and the peninsula of Lower California, the Pacific element is almost 



