. The Cyathaspididae; a family of Silurian and Devonian jawless vertebrates. Cyathaspididae. Fig. 133. Listraspis canadensis (X 2). A, incomplete dorsal shield, CNHM, PF 3683a; B, ventral shield, largely preserved as an impression, CNHM, PF 3687. orb, orbit; pip, postero-lateral point; ro, rostrum. by the lateral epitegum, and is situated about as is usual in cya- thaspids, that is, well in advance of the posterior end of the shield. The ventral surface of the brim of the dorsal shield continues as a distinct ventral lamina anterior to the branchial plate, under the orbits, and into the much n
. The Cyathaspididae; a family of Silurian and Devonian jawless vertebrates. Cyathaspididae. Fig. 133. Listraspis canadensis (X 2). A, incomplete dorsal shield, CNHM, PF 3683a; B, ventral shield, largely preserved as an impression, CNHM, PF 3687. orb, orbit; pip, postero-lateral point; ro, rostrum. by the lateral epitegum, and is situated about as is usual in cya- thaspids, that is, well in advance of the posterior end of the shield. The ventral surface of the brim of the dorsal shield continues as a distinct ventral lamina anterior to the branchial plate, under the orbits, and into the much narrower ventral lamina of the rostrum. Discontinuities in the ridge pattern of PF 3682 indicate that the suborbital part of this lamina, which shows pores of the infraorbital canal, is probably a distinct plate (fig. 132, sop) homologous to the suborbital plate of Anglaspis heintzi (fig. 94,A, sop). The part of the ventral lamina anterior to the suborbital plate is equivalent to the preorbital process of other cyathaspids. A small plate with a single lateral line pore (fig. 132,A, lap), lying just in front of the anterior end of the branchial plate, is comparable to a lateral plate of pteraspids. The internal impressions of pineal organ, brain, semicircular ca- nals, and gill pouches are much as in other cyathaspids (fig. 132,A). There are seven pairs of gill pouches (k2-k8 by Stensio's terminol- ogy), with k8 lying only slightly anterior to the branchial opening. The ventral shield (fig. 133,B) has a concave anterior border, and a strongly convex posterior border, the latter sometimes with a slight median lobe. In known specimens, this shield is only gently arched. The dentine ridge pattern is elliptical, except that the ridges flare anteriorly into a band of ridges parallel to the anterior Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not
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