153 GEOLOGICAL DISTRIBUTIOX. 



and here, also, for the first time, if we except the Laurentiaa rocks, 

 with the hypothetical Eozooii, do the Foraminifera appear to enter 

 largely as rock constituents. The genus Fusulina is developed to 

 an extraordinary extent, and its distribution appears to be but little, 

 if at all, less universal than that of the genera Nummulites and 

 Orbitoides of the Eocene period. A solitary forerunner of the 

 Nummulites has been discovered in the Carboniferous rocks of 

 Belgium. 



In the remarkable development of the elasmobranch (shark) 

 type of fishes, and in the absence of the bucklered ganoids, the 

 Carboniferous ichthyic fauna is sharply defined from that of the 

 Devonian. With the exception of a considerable number of fin- 

 spines or ichthyodorulites, referred to such genera as Ctenacanthus, 

 Gyracanthus, Oracanthus, &c., Avhose position is still very doubt- 

 ful, and which may in part belong to the order of ganoids, all the 

 remains of the former appear to have been more or less nearly 

 related to the modern Port Jackson sharks. These remains are in 

 the main in the form of teeth — Psammodus, Helodus, Orodus, 

 Chomatodus, Petalodus, Cochliodus — whose (somewhat distant) 

 resemblance to the pavement teeth of the cestracionts has led to 

 their reference to members of that group ; not impossibly, how- 

 ever, they represent a very distinct type.* The ganoids comprise, 

 in addition to polypteroid forms — Coelacanthus, Rhizodus, Megal- 

 ichthys — representatives of the rhomb-plated Lepidosteidei, which 

 include the American alligator-gar. The most widely distributed 

 and most abundantly represented genera are Palffioniscus and Am- 

 blypterus, the former of which is also one of the most abundant 

 fishes of the succeeding Permian period. 



The only vertebrates other than fishes which appear in the Car- 

 boniferous period, and now appear for the first time, are the Am- 

 phibia, that group of animals whose members stand immediately 

 next above the fishes in the scale of organisation, and whose em- 

 bryonic forms are so clearly ichthyic as to have necessitated the 

 union of the two classes into the one comprehensive division of the 

 Ichthyopsida. It is not a little significant that the appearance of 



* Mr. Garinan has recently described a species of shark from the Japanese 

 seas, Chlamydoselacbus anguineus, which appears to be generically most in- 

 timately related to the Carboniferous Didymodus, and which, accordingly, rep- 

 resents about the most ancient type among living vertebrates. 



