84: Dynamic Theory. 



poda) there are centipedes, also insects of the cockroach family. An air- 

 breathing G-asteropod ( Pupa, the land Snail), the Cyclas, a fresh water 

 bivalve, and the Cypris, a little Ostracoid Crustacean bivalve, make 

 their appearance. 



The Marine Vertebrates are of the same families of Selachians and 

 Ganoids, with many new and advancing forms, but there is an animal, 

 supposed to be a swimming lizzard, apparently foreshadowing the 

 Ichthyosaurus of the coming Jurassic period. Other remains are of 

 Batrachians and of a Salamandroid land animal or Amphibian, sup- 

 posed to be two and a half feet long. < ' The body was covered with 

 scales, and the whole surface of the cranium was sculptured. Dawson 

 regards it, therefore, as most nearly related to the Labyrinthodont. " * 



The amphibians are a type in advance of the fishes, and appear to 

 connect them on the one side, with the mammals, reptiles and birds on 

 the other. The Urodela, or tailed amphibians, represented by the Newt 

 (Triton), Salamander, Mud-eel (Siren}, Axolotl (Siredori), and Giant Sal- 

 amanders, are more or less fish-like. They all have gills to begin with, 

 and some retain them through life, others retain the gill openings but 

 lose the gills, while still others lose both and breathe through lungs. 

 The Anoura, or "tailless," include the frogs, toads, &c. They are an 

 advance on the tailed amphibians, and are, undoubtedly, derived from 

 them. In an extinct group of the tailed amphibians, there are scales, 

 which would indicate their fishy relationship. 



The embryo frog, while still within the egg, assumes the form of a 

 minute fish, devoid of limbs and with only rudiments of gills. After it 

 is hatched it gets three pairs of external gills, then these are covered 

 up, and internal gills grow from them. Then the lungs are developed, 

 and, for a time, the tadpole breathes through both lungs and gills. 

 The heart, at first, has two chambers. When mature the frog is devoid 

 of tail. His heart becomes three lobed, consisting of one ventricle and 

 two auricles. His lungs are in two lobes, right and left. He has kid- 

 neys, pancreas, spleen and liver, and a bilobed urinary bladder. He 

 has a brain with cerebral hemispheres, olfactory lobes and olfactory 

 nerves. He has a tympanic membrane, behind which is a tympanic 

 cavity connecting with the mouth by means of eustachian recess and 

 posterior nostril. But there is no external ear shell. Skull is partly 

 bony and partly cartilaginous. The tibia and fibula bones are fused 

 together, coracoid bone joins s"ternum and clavicle. He has a premax- 

 illary jaw bone. The duodenum forms a loop with the stomach. The 

 digestive, urinary and generative organs all discharge into a common 

 cloaca, which has one external opening. He is therefore monotrem- 

 atous, like the bird and ornithorhynchus. The fore limbs have four 



* An Amphibian. Dana 351. 



