460 Causes and Course of Organic Evolution 



the column are all similar. The centrum is perforate, and 

 the cavity is widely open, so that the notochord was very 

 little constricted. . . . The neural arch is free from the 

 centrum, and the two halves are separate. . . . There is 

 no trace of intercentra. The ribs are single-headed.'* The 

 relation of the bones in the skull, also, suggests ossifications 

 amid the cartilages typical of cyclostomes. The above are 

 all characters that would unite cyclostomatous with existing 

 ai)odous genera, and in this connection Case adds: "The 

 ai)i)arently limbless condition and the probable position of 

 the eyes far forward in the orbital spaces suggest some afiinity 

 with the limbless amphibians." 



The time and place of origin of the cartilaginous areas in 

 the cyclostome brain-box correspond very well with similar 

 areas in Apoda and Urodela where afterwards bone is depos- 

 ited. -But it should be said that it equally well corresponds 

 with what one finds for various groups of fishes, of reptiles, 

 and even of birds. As we shall shortly explain later, this 

 may be one of many cases of homoplasy or developmental 

 parallelism, that run through the entire organic world. Pos- 

 sibly however it may have a closer and more genetic explana- 

 tion than the writer is at present prepared to accept. 



The formation of two articular condyles for the skull is 

 typical of recent Apoda, is equally so of Urodela, and persists 

 up to the highest mammals. The ossification of the mandible 

 from three centers only — dentary, splenial, and angular — is 

 true alike of x\poda and Urodela, and is much simpler than 

 that of many fishes and specially of reptiles. The presence 

 of the dentary alone in mammals will later be dealt with. 



The numerous true teeth in Apoda are small, hard, conical, 

 ])ointed, and resemble each other, but show similar mode of 

 growth to that of higher craniates. Here a wide gap exists 

 i)etween the cyclostomes that have only horny epidermal 

 structures, and such perfected developments. Fossil remains 

 may yet bridge the gap however. The similar or homodont 

 teeth of Apoda and Urodela as compared with the usually 

 varied or heterodont teeth of Mammalia is another morpho- 

 logical gap in the record. But Wiedersheim's remarks (138: 

 245) are apropos: '*The heterodont dentition characteristic 

 of the Mammalia as a class must have arisen by a modification 

 of a simple homodont condition in which the teeth were all 

 conical, and of similar size and shape." 



The absence of limb-girdles and of limbs, ahke in Cyclo- 

 stomata and in Apoda, at times even in some Urodela, is a 



