Histologic studies of ostracoderms, placoderms and fossil elasmobranchs. 6. Hard tissues of Ordovician vertebrates

Size and distribution of dermal elements in Ordovician eriptychiids and astraspids are considered relative to phases of skeletal assimilation and regression. The phyletic significance of acellularity in aspidin is discussed, as also is the alleged relationship claimed to exist in initial developmental stages between that hard tissue and dentine proper. To judge from hard tissue histology the astraspids may not, like the eriptychiids, have belonged to the heterostracans but to another group, of early agnathans, still incompletely known.