11, NEW YORK STATE MUSEUM 



elements (commonly interpreted, however, as " premaxillaries " and "max- 

 illaries"), the latter with trenchant functional margin, often serrated or 

 denticulated. Lower dental plate, when ossified, intimately fused with the 

 forward portion of the mandible, turned in upright position with sharp cut- 

 ting edge, often serrated or denticulated like the upper. Symphysial 

 margin also sometimes denticulated. 



The structure of the typical genus, Coccosteus, is so well known from 

 the classic researches of Pander, Traquair and later writers, that further 

 description of it here would be superfluous. It furnishes a most valuable 

 standard of comparison for referring the detached plates of other Arthro- 

 dires to their proper position, and for correlating the numerous minor 

 variations observed in different members of the group. Interest centers in 

 it also from the fact that this is commonly looked upon as the progenitor 

 of later, much larger, and more highly specialized genera, occurring chiefly 

 in this country, the best known examples of which are Dinichthys and 

 Titanichthys. Owing to their intimate relations to Coccosteus, these two 

 genera are retained in the same family with it. We have already had occa- 

 sion to note the close structural agreement between Coccosteids and existing 

 Dipnoans, more particularly as regards their dentition and cranial osteology, 

 hence we need not dwell upon these points further in this connection. 



No traces of axial segmentation have been observed in any member of 

 this family, nor are there ossified ribs, Jaekel's representation to the con- 

 trary notwithstanding." There is one dorsal fin, the tail tapers gradually 

 and to all appearances was diphycercal, but definite information concerning 

 the caudal and anal is still lacking. Traces of pelvic fins occur, and a pair 

 of short deep plates lying immediately in front of the ventral armor in 

 Coccosteus has been interpreted as representing the pectoral arch. Never- 

 theless, positive evidence of the existence of pectoral fins has not as yet 

 been forthcoming, and the so called pectoral spine or " Ruderorgan " in 

 Coccosteus and Brachydirus is possibly the equivalent of the fixed spinous 



' Jaekel, O. Ueber Coccosteus und die Beurtheilung der Placoderraen, Sitzungsber. 

 Gas. Naturf. Freunde. 1902. p. 107, restoration. 



