APTYCHOPSIS WILSONI 105 



modified by pressure, but, if restored, would be circular. The notch is wide and 

 deep, rather more than half the length of the dorsal suture. The concentrics 

 numerous and narrow. In these two valves (somewhat obliquely squeezed) the 

 measurements are | , 11 mm.; — , 8 mm.; A, 6 mm. at 60°; ^_,, 6 mm.; I, 

 6 (?) mm. ; V , 60. Two valves, round-ended sagittate ; carapace circular ; width 

 and length about equal. 



This specimen, named A. glabra in 1872, has nearly the same proportions as 

 A. prima, var. secunda ; and is like A. Wilsoni, but is smaller, and has a relatively 

 smaller notch and no marginal rim. Although apparently smooth, it retains 

 indications of numerous fine concentric lines, as usual in Aptychopsis, and, in so 

 far, not quite in accordance with the smoothness intimated by " glabra." 



In bufi"-coloured, very finely micaceous shale. Valves brown, with a trace of 

 a black film, and with minute vermicular hematitic concretions. From the 

 Buckholm beds, Gala series ; Meigle, Galashiels, Selkirkshire. Coll. Lapworth. 

 Brit. Mus. No. 59620. 



6. Apttchopsis Wilsoni, H. Woodward, 1872. Plate XV, figs. 12 (?), 15, 16. 



Apttchopsis Wilsoni, E. Woodward. Geol. Mag., 1872, p. 565. 



— — — Report Brit. Assoc, for 1872 (1873), 



p. 323. 



— — — Catal. Brit. Foss. Crust., 1877, p. 70. 



— — T. B. J. and H. W. Geol. Mag., 1884, p. 354. 



— — — Report Brit. Assoc, for 1884 (1885), 



p. 89. 



— — Etheridge. Foss. Brit., vol. i, Palseoz., 1888, p. 41. 



This species has a discoidal shield, and was briefly described in 1872 

 as having an angular nuchal suture (making a triangular cephalic plate), and 

 a well-marked median or dorsal suture, and as measuring IJ inches in length 

 by 1-| inches across. There are three specimens of Aptijclwysis Wilsoni in the 

 British Museum, and they would probably be almost round in outline if quite 

 perfect. They are from the Riccarton beds (Wenlock beds, Upper Silurian), at 

 Shankend, Slitrig Water, near Hawick; Yad's Linn, near Hawick; and 

 Elliottsfield, near Hawick, Dumfriesshire. 



We may add that the cephalic notch is not so deep as in some allied forms ; its 

 apex was about one third of the length of the median suture from the front edge 

 of the shield. Concentric lines are apparent on some specimens. 



One large specimen would measure 40 mm. in each diameter if complete ; its 

 nuchal suture slopes 40°. Another specimen (imperfect) measures 30 mm. across, 



14 



