66 NEW YORK STATE MUSEUM 



Order CHIMAEROIDEI 



In all known Chimaeroids, whether recent or extinct, the notochord is 

 persistent and at most only partially constricted, the calcifications in the 

 sheath, when present, consisting of slender rings more numerous than the 

 neural and haemal arches. The pectoral fins are abbreviate, without seg- 

 mented axis ; and the pelvic fins in the male are produced into a pair of 

 claspers. In the extinct forms there is no trace of any dermal plate 

 developed in the opercular flap. The only clear evidence of evolution 

 hitherto observed concerns the development of the peculiar dental plates. 

 In each of the four known families the dentition consists of a few large 

 plates of vascular dentine of which certain areas (" tritors ") are specially 

 hardened by the deposition of salts within and around groups of medullary 

 canals, which arise at right angles to the functional surface. In most cases 

 there is a single pair of such plates in the lower jaw, meeting at the sym- 

 physis, while two pairs (the so-called vomerine and palatine plates) are 

 arranged to oppose these above. A permanent pulp remains under each 

 plate, and growth thus takes place continually within as the oral surface is 

 triturated by wear. In the surviving family of Chiniaeridae these dental 

 plates are much thickened, while the hinder upper pair (" palatines ") are both 

 closely apposed in the median line and considerably extended backwards. 



The dental plates named Ptyctodus, from the Devonic of Russia, the 

 Eifel district and North America, are essentially similar to those of modern 

 Chimaeroids, but there are no representatives of the vomerine pair. The 

 tritors, one only in each plate, are well differentiated, consisting of hard, 

 punctate, superimposed laminae, arranged obliquely to the functional sur- 

 face. The contemporaneous teeth known as Rhynchodus and Palaeomylus, 

 however, exhibit more indefinite tritoral areas, or none. The symphysial 

 facet is always distinct. 



.Spines which may be compared with those' of Chimaeroids are also 

 known from the Devonic and Carbonic systems, and Harpacanthus and 

 Cyrtacanthus may perhaps be cited as examples of head spines. No 



