584 



Arthrodires 



bearing jaw-tips of Diplognathtis, 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 



Fio. 368. — An Arthrodire, Dinichthiis intermedius Newberry, restored. Devonian, 

 Ohio. (After Dean.) 



Coccostciis, and recognizes as families the CoccosteidcB, Mylostomi- 

 dcr, Astcrosteida:, and PJiyllolcpidcc. 



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 Arthrognatlii, is first divided into two suborders, Auar- 

 tJirodira, without joint at the neck, and ArtJirodira, with such 

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

 latter two orders, Tcnuwtlioraci and ArtJirotlioraci. The foUowine 

 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 ha;mal elements. Paired limbs [absent or 

 uncertain]. Dermal plates consisting typically of two layers, 

 the superficial tuberculate, the inner bony with radiatino- la- 



* Memoirs New Yorlv Academy of Sciences, 1901. 



