72 IOWA GEOLOGICAL SURVEY 



oar-like swimming-organs are attached to the pectoral region, 

 but are of very different nature and structure from the fins of 

 gnathostomous fishes. Membranous median fins are developed, 

 however, the form of body is fish-like, there is a genuinely pis- 

 cine heterocercal tail, scales or scutes usually cover the ab- 

 dominal region, a well-defined sensory canal system is present 

 in the higher forms, and when the headshield is continuous it is 

 often pierced by branchial apertures. On account of these latter 

 characters we are warranted in postulating community of de- 

 scent between Ostracophores and the progenitors of ordinary 

 fishes. 



Of the four orders of Ostracophores now commonly recog- 

 nized, all have American representatives. The simplest forms 

 (Anaspida and Heterostraci) occur in the Upper Silurian and 

 Devonian, and are without paired appendages. Bone cells are 

 probably present in the calcifications of the Anaspida, but wholly 

 lacking in the Heterostraci. The third order, Osteostraci, is 

 confined for the most part to the Upper Silurian and Lower De- 

 vonian, though occasionally found in the Upper Devonian. Bone 

 cells are present, but there is no trace of dermal sense organs 

 either upon or within the shield. The fourth order, Antiarchi, 

 with a complex system of dermal plates and a remarkable pair 

 of appendages, is essentially Devonian, and as abundant in the 

 uppermost as in the lowest strata. The subclass becomes ex- 

 tinct at the close of the Devonian, without taking part in the 

 evolution of the fishes of later periods. 



Order HETEROSTRACI. 



These primitive Ostracophores are represented in the Palae- 

 ozoic rocks of North America by two genera, Cyathaspis and 

 Palseaspis, occurring in the Silurian, and by a variety of 

 Thelodus-like scales (PI. I, Figs, 7, 8, 15; PL II, Figs. 13, 14) 

 from the Lower and Middle Devonian. No indications of such 

 forms, however, have yet been found in States adjoining the 

 Mississippi Valley. The only American species of Cyathaspis 

 that has been recorded, C. acadica (Matthew), is founded upon 

 indifferently preserved material from strata of supposed Niag- 

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

 life yet discovered in Canada. Paloeaspis likewise appears to 



