32 NEW YORK STATE MUSEXJM 



Order ANXIARCHA 

 The Devonic Antiarcha possess a much more complex system of 

 dermal plates than other groups of Ostracophores, and are provided with a 

 pair of singularly jointed armored appendages, usually movable, evidently 

 serving as organs of progression, and totally unlike the limbs of other 

 vertebrates. The head and body shields are always movably articulated, 

 both being traversed by well defined sensory canals ; and the bone structure 

 is dense, though with vascular cancellae in the middle layer of the more 

 thickened plates. The external ornament always consists of tubercles and 

 coarse rugae. At least one membranous dorsal fin is present, and in Both- 

 riolepis there are two, according to the most recent accounts. The tail, 

 either naked or scaly, is furnished with a large membranous caudal fin of 

 genuinely heterocercal form. 



The forms belonging to this order are included within a single family, 

 the Asterolepidae, sometimes, but incorrectly called the " Pterichthyidae." 

 The typical genus, Pterichthys [text fig. 6], is not known to occur in this 

 country, but the Upper Devonic Bothriolepis, which difters from Pter- 

 ichthys principally in its longer appendages, scaleless tail, and various 

 details of the body armor, is found both in the eastern part of North 

 America and in Colorado, four species having been described in all. 

 Asterolepis itself appears to be represented by a single postero-dorsal plate 

 occurring in the Chapman sandstone of Aroostook county, Maine, the 

 accompanying invertebrates indicating a lowermost Devonic horizon. This 

 species, described in the sequel under the name of A. clarkei, in honor 

 of the present New York State Geologist, is noteworthy for continuing the 



