DEVONIC FISHES OF THE NEW YORK FORMATIONS 99 



been somewhat doubtfully interpreted as such by Newberry.' The pair 

 figured by him, however, have recently been ascertained to represent imper- 

 fect mandibular plates of an undescribed species of Mylostoma, and are 

 proved to have been firmly united in the median line. Under these con- 

 ditions intermandibular structures are definitely excluded. Amongst 

 Ctenodipterines, Synthetodus furnishes the only known example where 

 symphysial teeth are permanently retained, thus paralleling an evanescent 

 condition in Neoceratodus. Indeed, this very character leads one to sus- 

 pect that the form in question belonged to primitive Ceratodonts, rather 

 than to the Ctenodipterine order of Dipnoans. 



Systematic arrangement of Arthrodires. Various stages of evolution are 

 recognizable throughout the group, the most important being typified by 

 (i) Macropetalichthys, (2) Homosteus and (3) different genera comprised 

 by the families Coccosteidae, Mylostomatidae and Selenosteidae in order 

 of progressive specialization. In the system of classification proposed by 

 Bashford Dean, these stages find expression through the erection of three 

 new "orders," the first and most primitive of which includes Macropetali- 

 chthys and Asterosteus, the second Homosteus, and the third all of the 

 more highly specialized forms. Dean's arrangement of families, and for 

 the most part, of genera as well, is probably as judicious as present known 

 facts will allow ; hence we have adopted it in the following section of the 

 report, without, however, taking account of his larger divisions. His defi- 

 nition of one of them, as pointed out by Jaekel,"" includes structural charac- 

 ters which do not exist ; and we may be reasonably confident that in none 

 of them did the jaws operate by rotary movements, on which point Dean, 

 and following him Hussakof, lay great stress. 



Family ]VlA.CROE»BTA.LICHTHYtDAEJ 



Cranial shield much arched from side to side, completely inclosing the 



orbits, and extending over the nuchal region posteriorly. External surface 



covered with fine stellate tubercles which conceal the underlying sutures 



between dermal plates. Median series of plates but two in number, narrow 



'Newberry, J. S. U. S. Geol. Sur. Monogr. 1889. 16: 165, pi. 16, fig. 4. 

 *Jaekel, O. Referat in Neues Jahrb. f. Min. Jahrg. 1903. 1:342. 



