. The Cyathaspididae; a family of Silurian and Devonian jawless vertebrates. Cyathaspididae. Fig. 146. Homalaspidella borealis, dorsal shield (X 3). A, lateral view of right side, Princeton 17379; B, rostral view of type, Princeton 17101. bro, branchial opening; orb, orbit; ro, rostrum. Discussion.—Though older than H. nitida, this species appears to be more advanced in the simplification of the dorsal shield out- lines and of the dentine ridge pattern. The orbital notches and preorbital processes (fig. 146,A) are so slightly developed that it is difficult at first to recognize a dorsal shield
. The Cyathaspididae; a family of Silurian and Devonian jawless vertebrates. Cyathaspididae. Fig. 146. Homalaspidella borealis, dorsal shield (X 3). A, lateral view of right side, Princeton 17379; B, rostral view of type, Princeton 17101. bro, branchial opening; orb, orbit; ro, rostrum. Discussion.—Though older than H. nitida, this species appears to be more advanced in the simplification of the dorsal shield out- lines and of the dentine ridge pattern. The orbital notches and preorbital processes (fig. 146,A) are so slightly developed that it is difficult at first to recognize a dorsal shield by its shape. In addi- tion, the absence of epitega and of a pineal macula, and the presence of longitudinal ridges covering most of the shield (fig. 145), make recognition even more difficult. The transverse commissures and medial longitudinal canals of the dorsal sensory line system are di- vided into short lengths. The ventral shield (fig. 147) is 25-29 mm. long, and has a width ratio of .58. Its ridge pattern resembles that of the dorsal shield: there are a few anterior transverse ridges that con- tinue around to the lateral margins, and the central ridges are longi- tudinal, becoming irregular behind the anterior transverse ridges. Ariaspis Denison Type-species.—Ariaspis ornata Denison. Ariaspis Denison, 1963, Fieldiana: Geol., 14, no. 7, pp. 120-123, 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 Museum Press
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