CRETACEOUS CHIM^ROIDS. 



149 



gill-arches. This view, it will be seen, finds some support in the remarkable gill- 

 arch-like character of the labial cartilages, and it becomes less fanciful when one 

 considers how frequently the labial cartilages, especially in Chimseroids, have been 

 homologized with premandibular arches. As far as Myriacanthus is concerned, 

 such interpretations are clearly favored by our knowledge of its evident speciali- 

 zation in dermal defenses, for in such a light it would be not improbable that addi- 

 tional dermal elements would be evolved and impressed into the service of the 

 mouth parts/, e., plates which may not have been present in the parent stock 

 from which descended Myriacanthus, Squaloraja, and modern Chimseroids. 



Moreover, it is worthy of mention that the forms which are commonly accepted 

 as the earliest Chimseroids, the Ptyctodontids, have but two pairs of dental plates. 

 For it might be plausibly suggested that these primitive forms had not reached the 

 stage in evolution when the "vomerines" (i. e., dermal elements) appeared as 

 defenses for the anterior arch. 



In accordance with the present considerations a scheme of the evolution of the 

 genera of Chimseroids may be arranged somewhat as on page 150, fig. 144. 



