28 NEW YORK STATE MUSEUM 



specially related to Ascidians, Crustaceans or Arachnids, this seems to 

 be clearly disproved by the following well known facts: (i) the dermal 

 plates are composed of true bone; (2) the head shield and body armor of 

 Asterolepids have a well developed sensory canal system ; ( 3 ) in Pterichthys, 

 at least, there is a tail covered with scales, a dorsal fin, anci a genuinely 

 piscine heterocercal caudal fin. 



Of the four orders of Ostracophores now commonly recognized, all 

 have American representatives. The simplest forms (Anaspida and Heter- 

 ostraci) occur in the Siluric and Devonic, and are without paired append- 

 ages. Bone cells are probabl)' present in the calcifications of the Anaspida, 

 but wholly lacking in the Heterostraci. The third order (Osteostraci) is 

 confined, as a rule, to the Siluric and Lower Devonic, though occasionally 

 met with in the Upper Devonic. Bone cells are present, but there is no 

 trace of dermal sense organs either upon or within the shield. The fourth 

 order (Antiarcha), with a complex system of dermal plates and a remarkable 

 pair of appendages, is essentiall)- Devonic, and as abundant in the upper- 

 most, as in the lowest strata. The subclass becomes extinct at the close of 

 the Devonic, without taking part in the evolution of the fishes of later 

 periods. 



Order HETEROSXRACI 



These primitive Ostracophores are represented in the Paleozoic rocks 

 of North America b)' two genera, Cyathaspis and Palaeaspis, occurring in 

 the Siluric, and by a variety of Theloduslike scales from the Lower and 

 Middle Devonic. No indications of sucli forms, however, have yet been 

 discovered within the limits of New York State. The only American 

 species of Cyathaspis that has been described, C. acadica (Matthew), is 

 founded upon indifferently preserved material from strata of supposed 

 Niagara age in New Brunswick, and is the oldest trace of vertebrate life 

 yet discovered in Canada. Palaeaspis likewise appears to be known in this 

 country by a solitary species occurring in the middle portion of the Salina 

 beds in Perry count)- Penns)lvania. In view of the confiicting statements 

 which exist in the literature concerning the sj'nonjm)', geological horizon 



