. The Cyathaspididae; a family of Silurian and Devonian jawless vertebrates. Cyathaspididae. DENISON: THE CYATHASPIDIDAE 389. -/—pbl Fig. 130. Pionaspis acuticosta, restoration of dorsal shield based on type (X 1). A, dorsal view; B, lateral view of right side. bro, branchial opening; cdi-z, dorsal transverse sensory commissures; cso, trans- verse supraorbital sensory commissure; Idl, mdl, lateral and medial dorsal sensory canals; orb, orbit; pbl, postbranchial lobe; soc, supraorbital sensory canal. lareaspidinae in being relatively short and deep, with the anterior edges abruptly truncate. Ho
. The Cyathaspididae; a family of Silurian and Devonian jawless vertebrates. Cyathaspididae. DENISON: THE CYATHASPIDIDAE 389. -/—pbl Fig. 130. Pionaspis acuticosta, restoration of dorsal shield based on type (X 1). A, dorsal view; B, lateral view of right side. bro, branchial opening; cdi-z, dorsal transverse sensory commissures; cso, trans- verse supraorbital sensory commissure; Idl, mdl, lateral and medial dorsal sensory canals; orb, orbit; pbl, postbranchial lobe; soc, supraorbital sensory canal. lareaspidinae in being relatively short and deep, with the anterior edges abruptly truncate. However, Pionaspis apparently lacks the other characteristic features of that subfamily—the very fine den- tine ridges, the much branched lateral line canals, and the posteri- orly placed orbits. As far as is known, there is nothing to debar Pionaspis from the subfamily Cyathaspidinae. As is commonly the case in Vernonaspis, two species of Pionaspis are found together. The length of the dorsal shields is about the same in the two species, but the breadth of the dorsal shield of P. acuticosta is considerably greater. The type of the latter is crushed and somewhat flattened, but this cannot account for the difference in the width ratio. A transverse measurement following the surface of the shield across the postbranchial lobes is about 75 mm. in P. acu- ticosta, but only about 60 mm. in P. planicosta. Moreover, the dif- ference in shape and coarseness of the dentine ridges indicates that these are two distinct species. The dorsal shield is only slightly arched. The median rostral process is absent, but there is a slight median convexity on the ros-. 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 perfectly resemble the original Denison, Robert H. (Robert Howland), 1911-1985; Chicago Natural History Museum. Chicago : Chicago Natural History
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