\66 



THE OSTRACODERMS. 



lateral eye openings. Gills enclosed in peribranchial, or atrial chamber. Mouth 

 parts unknown. Body plates small, rhomboidal. 



Pteraspidae. Dermal armor ornamented with minute dentinal ridges and 

 grooves, parallel with the margins of the various plates. Matrix sharply laminate, 

 with numerous unbranched, parallel pore canals, terminating in spindle-shaped 

 unipolar lacunae. Cancellae large, rectangular, and in a single layer. Parietal 

 eye lodged in a hollow tubercle, prominent externally, and with the cavity opening 



B 



FIG. 246. Restoration of Drepanaspis gemundenensis, Schl. (After Traquair; slightly modified.) A, Haemal 



surface; B, neural surface. 



inw r ard. Cephalic appendages, where known, large, armored. Oblong marginal 

 openings on dorsal surface of cornual plates leading into the interior of the head. 

 Upper Silurian and lower Devonian. Six to nine inches long. Pteraspis. Kner. 

 (Fig. 245.) Palaeaspis, Claypole. Oldest member of the ostracoderms known to 

 occur in America. Onondaga Group, Perry Co., Penn. (Fig. 244, B and C.) 

 Cyathaspis, Lank. (Fig. 244, A.} 



Psamostaedae. Head, broad, flattened; trunk, short and thick. Ornament 

 minutely tubercular. Large plates of the head separated by rows of small, 



