Ii6 TYPES OF ANIMAL LIFE 



When, however, we descend to the lias and trias and 

 carboniferous rocks we come upon a great variety of 

 extinct species of animals which have been hitherto re- 

 garded as being nearly allied to the three batrachian orders 

 still existing. They constitute another — fourth — order, 

 to which the term Labyrinthodonta has been applied 

 for a reason which will be stated shortly. Thus 

 our search into the past has brought us a rich and 

 important gain. The i.abyrinthodonts were creatures 

 with long tails and mostly two pairs of limbs, but 

 their limbs were always of relatively small size. Some 

 species were very large, even exceeding the great sala- 

 mander of Japan in size. Some of them had large 

 and formidable teeth in long elongated jaws like those 

 of alligators, and the structure of their teeth is very 

 noteworthy. They are conical in shape and marked 

 superficially by slight vertical grooves. If we make a 

 horizontal section of one of their teeth we shall then see 

 that these surface grooves are the external indications 

 of deep indentations of the substance of the tooth. All 

 these indentations converge toward the central long axis 

 of the tooth, but do not converge in straight lines, each 

 indentation being elaborately inflected. Radiating from 

 the central axis of the tooth we shall find in our section 

 a corresponding number of grooves radiating outward 

 from the tooth's central pulp cavity, these radiating folds 

 passing between, and being inflected in undulations like, 

 the converging grooves. Such a tooth is a beautiful 

 object when examined with a good magnifying glass, and 

 the markings thus produced by so many radiating and 

 converging folds (undulating and alternating in regular 

 order) are so complicated and labyrinthic, that labyrinthic 

 tooth becomes an appropriate name for the members of 

 this singular order. Its members were doubtless aquatic 



