112 INTRODUCTION TO CLASSIFICATION. 



3. The GrYMNOPHiONA have rounded, worm-like bodies, which 

 are devoid of lirnbs and tail. They have scales imbedded in the 

 integument. The dorsal vertebrae are bi-concave, and possess 

 double transverse processes, with which the capitula and tu- 

 bercula of the ribs articulate. 



The genera Csecilia, Siphonops, Ichthyophis, and Rhinatrema 

 belong to this order. 



4. The LABYRINTHODONTA. The body is salamandriform, 

 with relatively weak limbs, and a long tail. The dorsal 

 vertebrae, when completely ossified, are bi-concave, with double 

 transverse processes. The ribs have distinct capitula and 

 tubercula. 



In the thoracic region, three superficially sculptured exo- 

 skeletal plates, one median and two lateral, occupy the place of 

 the interclavicle and clavicles. Between these and the pelvis is 

 a peculiar armour, formed of rows of oval dermal plates, which 

 lie on each side of the middle line of the abdomen, and are 

 directed obliquely forwards and inwards, to meet in that 

 line. 



The skull has distinctly ossified epiotic bones in the same 

 position and of the same form as those of fishes. The cranial 

 bones are sculptured, and many exhibit peculiar smooth, sym- 

 metrical grooves the so-called " mucous canals." 



The parietes of the teeth are deeply plaited and folded, so 

 as to give rise to a complicated " labyrinthine " pattern in the 

 transverse section of the tooth. 



Remains of the Labyrinthodonts, which sometimes attained a 

 large size, are found from the Carboniferous to the Triassic 

 or Liassic strata, inclusively. 



The class PISCES is divisible into six orders 



1. DIPNOI. There is a skull with distinct cranial bones, and 

 a mandible. The notochord is persistent, and there are no 

 vertebral centra. 



There are two pairs of filiform fins, each supported by a 

 single, jointed, cartilaginous rod. The posterior pair are placed 

 close to the anus. The pectoral arch has a clavicle. 



The heart has two auricles, and true lungs coexist with 



