406 STUDIES FOR STUDENTS 



be preserved through the accidents of fossilization more often 

 than the softer structures of the skeleton. 



The Rays or Batoid branch of the Elasmobra?ichii are of 

 doubtful origin, other than the fact that they differ from the 

 true sharks only in the modifications that are attendant on the 

 extreme flattening of the body due to the habit of bottom feed- 

 ing. They probably originated as far back as the Carboniferous 

 time, although well-preserved forms are not known from rocks 

 earlier than the Jurrassic. Rhinobatis, an existing genus, is 

 known from the Oolite. 



The part played by the sharks in the waters of the Palaeozoic 

 oceans seems to be very much the same as that played by the 

 recent bony fishes in the modern waters. The variety of forms 

 was seemingly endless, and the adaptations to various conditions 

 of life, and to different means of obtaining food, bear witness, 

 not only to the large number of forms, but to the strength 

 and dominance of the type. 



The Holocephali, the Chimeroids, is a peculiar, aberrant 

 group, that is allied on the one hand to the sharks and on the 

 other to the Dipnoans. The structure of the fins and the verte- 

 bral column is the same as in the sharks, but the structure of 

 the head and the arrangement of the teeth is almost the same 

 as the dipnoans. There is a single broad dental plate in the 

 lower jaw, and two plates in the upper jaw instead of numerous 

 isolated teeth. The surface of these tooth plates is variously 

 modified by the addition of knobs, ridges, and other irregular- 

 ities, to increase the grinding- surface. Little is left of the fossil 

 forms but the teeth and the spines that stood at the anterior 

 end of the dorsal fin as in the sharks. Fossil forms are Ischyodus, 

 Myriacantkus, and Sqitaloraja, all from the Mesozoic. 



The Dipnoi are, perhaps, the most peculiar of all of the fishes. 

 For a long time considered as the linking type between the 

 fishes and the amphibians, they are now generally regarded as 

 a degenerate group that originated from the stem of the Cross- 

 opterygian Teleostomes. They are characterized by the absence 

 of a connecting bone, the quadrate, between the skull proper 



