120 IOWA GEOLOGICAL SURVEY 



indeed go so far as to conclude that the ancestral Holocephali 

 diverged from the Selachian stem near or even within the group 

 of the Palaeozoic Cestracionts. Indeed, the recent Chimaeroids 

 and Cestracionts retain many features of kinship. Among these 

 need only be mentioned at the present time approximations in 

 dentition, labial cartilages, articulation of mandibles, structure 

 of fins, and urogenital system. Even the complicated egg-cap- 

 sule of Chimaeroids finds its nearest parallel in the recent Ces- 

 traciont, a comparison often lost sight of on account" of the 

 spiral arrangement of the lateral webs in the capsule of the latter 

 form. 



From the standpoint of taxonomy, on the other hand, it must 

 be clearly recognized that the Chimaeroids have been separate 

 from the early sharks for so long a time and have acauired such 

 different characters that they are to be given a high rank among 

 the divisions of the subclass Elasmobranchii, the equivalent, lei 

 us say, of such groups as Pleuracanths or Pleuropterygians. 



Family PTYCTODONTIDAE. 



A family at present indefinable, of doubtful ordinal position, 

 known only by remains of the dentition and associated dermal 

 ossifications. A single pair of large, laterally compressed dental 

 plates present in each jaw, united but not fused at the symphysis, 

 and either with trenchant oral margin, or developing: one or 

 more tritoral areas posteriorly. 



Rhynehodus undoubtedly represents the most primitive condi- 

 tion of dental plates, the tritors of Ptyctodus and Palaeomylus 

 arising at a subsequent stage, and indicating greater specializa- 

 tion. The two last-named genera are exclusively Devonian, 

 Rhynehodus alone persisting as late as the TVaverly (Lower 

 Carboniferous). 



Ever since Pander's investigations of Ptyctodus, in 1858, the 

 opinion has prevailed among authors generally, until very recent 

 times at least, that the Ptyctodont type of dentition affords posi- 

 tive indications of Chimaeroid or Chimaeroid-like fishes during 

 the Palaeozoic. So far as reliance can be placed upon detached 

 dental elements, unaccompanied by other parts of the skeleton, 

 this determination of their nature is eminently justifiable, inas- 

 much as no essential differences are to be observed between the 

 dental plates of Ptyctodus and its congeners, and those of mod- 

 ern Chimaeroids. Palaeoniylus differs from the typical genus 



