FOSSIL VERTEBRATES — FISHES 4<=>7 



and the lower jaw. This condition, called autostylic, is found 

 in the preceding group. Other characters are the modifica- 

 tion of the swim bladder, in the living forms at least, as a 

 breathing organ that assumes to some extent the structure of 

 the lungs of the land vertebrates, the presence of tooth plates 

 instead of teeth in the jaws, the archipterygial structure of 

 the fins and the diphycercal tail. At the present time there 

 are only three genera of the group living, but their wide dis- 

 tribution points to a very great development in earlier times. 

 The existing genera are Protopterus from Egypt, Ceratodus from 

 Australia, and Lepidosiren from South America. In these forms, 

 as in the fossils, the vertebrae are incompletely ossified, the 

 centrum remaining cartilaginous, while the upper and the lower 

 arches are fairly well ossified. The major portion of the 

 skeleton is well ossified, thus showing a condition in advance of 

 the sharks and Chimeroids. 



The Dipnoi are an exceedingly ancient group ; even as early 

 as the Devonian they had developed most of the characters that 

 distinguish them from the other fishes. While it is altogether 

 probable that they originated from the primitive shark stem, the 

 point of origin seems to be totally lost, for the well-developed 

 Dipnoans are found contemporaneous with the earliest of the 

 sharks. Of the early Dipnoi, perhaps the most interesting is 

 Dipterits from the Old Red Sandstone. This form had well- 

 developed cycloidal scales, the type of the modern fish scale, 

 while the sharks still had the shagreen denticles, and the Ostra- 

 coderms the thick, bony, rhomboid scales so characteristic of 

 the early fishes. It had the skull protected by a roofing of 

 dermal plates, and the teeth modified into large plates, with 

 rough triturating surfaces. The plates were arranged as in the 

 Holocephali, two in the upper jaw and one in the lower jaw of 

 each side. 



Phdneropleuron, from the same horizons in Scotland, is more 

 primitive in some of the characters ; for instance, the jaws are 

 provided with many small, conical teeth, and the dorsal fin is 

 continuous instead of being broken up into two or three seg- 



