. Bulletin of the British Museum (Natural History), Geology. 82 PTERASPIS LEATHENSIS WHITE, A DITTONIAN ZONE-FOSSIL from the 'Passage Beds' (Psammites de Lievin) of the Pas-de-Calais (see Barrois, Pruvost, & Dubois, 1922: 180-4). Thanks to the kindness of Professor Leriche and Professor Pruvost I have been able to examine these specimens from the collections of the University of Lille. There are four dorsal shields, two of which are nearly com- plete (Text-figs. 15-17, 19), but the surface of the plates has almost disappeared and. Pteraspis [Simopteraspis) leathensis White Text-fig. 10. Im
. Bulletin of the British Museum (Natural History), Geology. 82 PTERASPIS LEATHENSIS WHITE, A DITTONIAN ZONE-FOSSIL from the 'Passage Beds' (Psammites de Lievin) of the Pas-de-Calais (see Barrois, Pruvost, & Dubois, 1922: 180-4). Thanks to the kindness of Professor Leriche and Professor Pruvost I have been able to examine these specimens from the collections of the University of Lille. There are four dorsal shields, two of which are nearly com- plete (Text-figs. 15-17, 19), but the surface of the plates has almost disappeared and. Pteraspis [Simopteraspis) leathensis White Text-fig. 10. Imperfect left flank-scale with exceptionally large area of overlap. [] Text-fig. ii. Right flank-scale, probably from near top of series. [] Text-figs. 12, 13. Double flank-scale, probably from right flank and therefore covering two diagonal rows, but orientation not certain. If inverted each would cover the area of two scales in the same row. In Fig. 13 the area of overlap has been broken away. [, RP700.] Text-fig. 14. Anterior ridge-scale. [RP718.] All specimens from Porch Brook, x 8. very little is left of the ornamentation, so that the outlines of the individual plates are most difficult to determine, especially in the pineal and orbital region. The largest is rather smaller than the Ammons Hill specimens, having a median length of 40 cm., while the smallest is about the size of the Porch Brook series. There seems little or no difference between the French and English specimens in proportions when allowance is made for curvature, but so far as one may judge, the former have a relatively larger pineal plate, a larger base of the dorsal spine, and shorter cornual plates, while the rostrum seems more acute (cf. Text-figs, ig-20). It is, however, not impossible that when well-preserved specimens of P. gosseleti are forthcoming the two forms may prove to be conspecific. Pteraspis vogti has not yet been described and our published knowledge of it is confined
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