584 Arthrodires 



bearing jaw-tips of Diplognathus, to the Cestraciont type, Mylo- 

 stoma. The latter form has hitherto been known only from its 

 dentition, but now proves to be, as Newberry and Smith Wood- 

 ward suggested, a typical Arthrodiran." 



Classification of Arthrodira. Our knowledge of the system- 

 atic relations of the Arthrodira is mostly of recent origin. 

 Woodward refers most of the remains to the best known genus 



FIG. 368. An Arthrodire, Dinichthvs intermedius Newberry, restored. Devonian, 

 Ohio. (After Dean.) 



Coccosteus, and recognizes as families the Coccosteida, Mylostomi- 

 d<z, Asterosteidce, and Phyllolepidce. 



Dr. Bashford Dean in different papers has treated these 

 fishes in great detail. In a recent paper on the "Relationships 

 of the Arthrognathi " * he recognizes the group as a class coor- 

 dinate with Cyclostomi and Elasmobranchii. This class, which 

 he calls Arthrognathi, is first divided into two suborders, Anar- 

 throdira, without joint at the neck, and Arthrodira, with such 

 a joint. The former comprises one order, Stegothalami, and the 

 latter two orders, Temnothoraci and Arthrothoraci. The following 

 is Dr. Dean's definition of these orders and their component 

 families : 



Arthrognathi.- "Chordates whose anterior body region is 

 encased in dermal elements, and divisible by a more or less 

 definite partition into head and trunk. Dermal plates which 

 surround the mouth function as jaws. No evidence of branchial 

 arches. Column notochordal, showing no traces of centra ; well- 

 marked neural and haemal elements. Paired limbs [absent or 

 uncertain]. Dermal plates consisting typically of two layers, 

 the superficial tuberculate, the inner bony with radiating la- 



* Memoirs New York Academy of Sciences, 1901. 



