. The Cyathaspididae; a family of Silurian and Devonian jawless vertebrates. Cyathaspididae. 460 FIELDIANA: GEOLOGY, VOLUME 13 units within the areas that are designated epitega, but this may not have been true for deeper layers. This manner of growth is very different from that of Pycnaspis and from that inferred by Stensio for Fig. 159. Transverse sections of dermal shields of juvenile cyathaspids (X 75). A, Cyathaspis cf. acadica, CNHM, slide 4028; B, Allocryptaspis laticostata, CNHM, slide 4043. as, aspidine; dn, dentine; dru large elevated dentine ridge; ig, intercostal groo
. The Cyathaspididae; a family of Silurian and Devonian jawless vertebrates. Cyathaspididae. 460 FIELDIANA: GEOLOGY, VOLUME 13 units within the areas that are designated epitega, but this may not have been true for deeper layers. This manner of growth is very different from that of Pycnaspis and from that inferred by Stensio for Fig. 159. Transverse sections of dermal shields of juvenile cyathaspids (X 75). A, Cyathaspis cf. acadica, CNHM, slide 4028; B, Allocryptaspis laticostata, CNHM, slide 4043. as, aspidine; dn, dentine; dru large elevated dentine ridge; ig, intercostal groove; pu, pulp cavity; tb, tubercle. There is some question whether the ventral shield was subdivided into hypotega equivalent to the epitega of the dorsal shield. Cyath- aspis banksi (fig. 110,B) has a band of fine ridges parallel to the an- terior edge and to the anterior parts of the lateral edges; these were designated by Stensio (1958, p. 308) "anterior postoral" and "mar- ginal" hypotega. Similar bands are formed in Cyathaspis acadica (fig. Ill) and in an undetermined cyathaspid from southeastern Yukon (Princeton 17094). But there are comparable ridges parallel to the anterior and lateral edges of both the dorsal and ventral shields of Homalaspiclella (figs. 145, 147), a genus that has gone farther than any other cyathaspid in the reduction of epitega. This and the ab- sence of any apparent subdivision of the ventral shield in most cyath- aspids makes the presence of distinct hypotega seem improbable. The anterior and posterior "hypotega" that Stensio (1958, p. 316) described in Anglaspis insignis have been shown above (p. 428) to be merely the result of an individual variation in ridge pattern. Within the Cyathaspididae, the final evolutionary stage appears to be the loss of most or all traces of epitega. It is this feature espe- cially that characterizes the Poraspidinae, and it is found also in the later genera of the Irregulareaspidinae. Th
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