414 ROSS ox EVOLUTION. 



One jaw is straight and the other like a bow, and while one contains 

 from four to six teeth, the other contains from thirty to thirty-five." 



It is interesting to note the development of true teeth among the 

 oldest, so far as is known, of Vertebrates — the Skarks, for while 

 the earliest had pavement teeth — broad plates like the dermal plates 

 of their contemporaries the Ganoids, other Families of Sharks in the 

 Carboniferous had narrower and sharper teeth, while in some exist- 

 ing Families they are quite sharp. In the Ganoids the dermal plates 

 vary much in character and disposition. Some of them had pave- 

 ment teeth. The Sturgeon, an existing Ganoid, has no teeth. 

 Most Ganoids since about the close of the Palaeozoic Period, have 

 shortened and bilobed or hetrocercal tails when mature, but when 

 young have homocercal tails, like the ancient Ganoids. The Ptero- 

 sauria, higher Batrachia, later A.ves, higher Simiadce, and the 

 Ajnth7'opidce, have the tail completely atrophied. 



The Teleosts deposit their spawn before fecundation takes place, 

 while the Shark is in fact placental, bringing forth its young in a 

 well advanced condition. Other existing Orders, of which however 

 there are but few living representatives, show how exceedingly 

 varied and wide in its limits is the organization of Fishes, for while 

 the '*Mud Fish" has a heart with two auricles, external rudimentary 

 branchiae, internal functional branchiae, and true lungs, being thus 

 much above the ordinary level of Fishes, the Amphtoxus lanceolatus 

 has no heart but only contractile arteries, no kidneys, a sac like liver, 

 no vertebral arches, no distinct brain, no auditory organs, neither 

 a cartilaginous nor an osseous skull, nor a mandible, nor any limbs, 

 and even the Order represented among living Fishes by the Lampreys 

 and Hags, though much more highly organized than the last men- 

 tioned, seem devoid of any indurated tissue. And here I would 

 remark the great imperfection of the Geological Record, since 

 generally speaking only highly indurated tissue could be preserved, 

 and thus whole Orders, even, of Cartilaginous Fishes have probably 

 perished, leaving no fossil trace, and if they should happen to have 

 no living representatives, as is known to be the case with some 

 Orders among the Reptilians, then no definite record of their 

 existence may now remain. And if whole Orders have no fossil 

 representative, because devoid of well indurated tissue is it not 



