Arthrodires 



S^5 



mellae. Orbits situated near or at the margin of the head-shield 

 and separated from one another by fixed integumental plates. 

 A pineal funnel present situated in a fixed plate. A mucous 

 system whose canals radiate from the preoccipital region." 



Anarthrodira. — " Arthrognaths in which the cranial and 

 dorsal regions are separated by a fixed partition whose dorsal 

 rim is overlapped and concealed by superficial plates. Of these 

 a large median dorsal element is present which extends back- 

 ward superficially from the region near the pineal funnel. Also 

 a pair of elements which overlie the position of the external 

 occipital joint. Suborbital plates apparently absent. Jaw 

 elements undescribed." 



Stegothalami (crre;/oj, roof; ddXa/nos, chamber). — "Anarthro- 

 dires in which the cranio-dorsal septum is vertical and deep, 

 its height equal apparently to that of the arch of the head- 

 shield. By this deep partition the latter appears to inclose two 

 chambers (whence the ordinal name). Orbits inclosed by pre- 

 and postorbital plates, ilucous system lacks a postorbital 

 canal." 



One family, the Alacropetalichthyidcs, thus defined: 



"Stegothalami with large orbits and well-arched cranio- 

 dorsal shield. Dorso-central shield long, wide, gomphoidal, 

 extending backward to the hinder margin of the shield and 

 bordered by all plates save the postorbitals and marginals. 

 Pineal funnel small and obscure." Macro petalichthys sullivanti 

 from Ohio Devonian rocks, and AlacropetalichtJiys agassizi from 

 the Devonian of Germany, are important species of this 

 group. 



The Asierosteidcc perhaps constitute a second family in 

 this order. The single species Asterosteus stenocephalits is from 

 the Devonian of Ohio. 



Arthrodira. — "Arthrognaths in which the dorsal armoring 

 is separated into dorsal and cranial elements, the latter attached 

 to the former movably by means of a pair of peg-and-socket 

 joints. The interval lying between cranial and dorsal armor- 

 ing does not appear to have been protected by plates, and in 

 the median line, instead of the cranio-central of the Anarthro- 

 dires, there are separate elements, median occipital, median dorsal, 

 and perhaps others. Suborbital plates present. Jaws of three 



