. Bulletin of the British Museum (Natural History), Geology. 8o PTERASPIS LEATHENSIS WHITE, A DITTONIAN ZONE-FOSSIL its absence in the numerous specimens of P. rostrata (White, 1935) is due to the chances of preservation—which is rather remarkable in view of the superb state of preserva- tion of the specimens from Wayne Herbert and Trimpley. Recently a fresh examina- tion has shown that this feature is partly preserved in one of the Trimpley scales ().. Pteraspis [Simoptevaspis) leathensis White Text-fig. 5. External impression of pineal area of dorsal shield. Ammons Hill. [P-23793- X 2
. Bulletin of the British Museum (Natural History), Geology. 8o PTERASPIS LEATHENSIS WHITE, A DITTONIAN ZONE-FOSSIL its absence in the numerous specimens of P. rostrata (White, 1935) is due to the chances of preservation—which is rather remarkable in view of the superb state of preserva- tion of the specimens from Wayne Herbert and Trimpley. Recently a fresh examina- tion has shown that this feature is partly preserved in one of the Trimpley scales ().. Pteraspis [Simoptevaspis) leathensis White Text-fig. 5. External impression of pineal area of dorsal shield. Ammons Hill. [P-23793- X 2.] Text-fig. 6. Rostral area of small dorsal shield (see also PI. 5, Fig. i). Porch Brook. [ X 2.] Text-fig. 7. Left side of dorsal shield of abnormal, blind specimen without orbit and with hinder part of orbital plate fused with dorsal disk. Porch Brook. [ X 3 approx.] Text-fig. 8. Imperfect dorsal spine and socket, in dorsal and (a) lateral views. Porch Brook. [ x 2.] The ornamentation follows the usual pattern of the genus (PL 5, Figs, i, 2) and comes within the usual range of fineness (50-80 ridges per cm.; see White, 1938: 107). The individual ridges are A-shaped in section with fine but conspicuous denticulation when unworn. A feature of this species is the marked irregularity of the ridges at the beginning of the later growth-stages, especially on the dorsal and ventral disks, as indicated by the extreme unconformity between the ridges outside the major-growth lines. In the holotype (Text-fig. 2) two such stages are shown, in the large disks from Ammons Hill only one (Text-figs. 3, 4, 9), but in the little specimen from Porch Brook (PL 5, Fig. i) there is none, which suggests that it is not fully grown. These. 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 British Museum (Na
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