200 Sedgwick Museum Notes — 



II. — Sedgwick Museum Notes. 



By F. E. COWPER Eeed, M.A., F.G.S. 



(PLATE XI.) 



DioNiDE ATRA, Salter. (PI. XI, Figs. 1-6.) 



In 1866 Salter^ established this species on the basis of two some- 

 what distorted pygidia, but the liead-shield and thorax have remained 

 undescribed and apparently unknown. The discovery therefore by 

 Mr. V. M. Turnbull of some complete though somewhat imperfectly 

 preserved individuals in the Arenig Beds near Haverfordwest is of not 

 a little interest ; and with the aid of several detached head-shields, in 

 addition to those belonging to individuals with typical pygidia, we are 

 able to give a fairly complete description of the species, which, owing 

 to its relations to other genera, is particularly worthy of note. 



Description. Head-shield semicircular or semi-elliptical, gently 

 convex, with genal angles produced broadly backward and pointed. 

 Border narrow, smooth, widest in front, narrowing posterioiiy on each 

 side, separated from rest of head-shield by narrow marginal furrow 

 in which lies a single row of equidistant and equal elongated pits. 



Cheeks gently convex, divided into (1) an outer circumferential 

 region, and (2) an inner trianguhir region on each side of glabella, and 

 connected in front of it by a narrow band. Circumferential region 

 produced back into genal prolongations ; marked off from inner region 

 by possessing a slight independent convexity and by a different 

 ornamentation ; internally defined by three concave curves, two 

 lateral and one median, meeting on each side in obtuse angles a little 

 behind anterior end of glabella ; median part of circumferential region 

 of uniform width ; lateral parts widest at obtuse angles of Junction, 

 and sharply bounded behind by fine raised vascular trunk running 

 backwards towards genal angles. Surface of circumferential region 

 marked by radiating sinuous branching and anastomosing fine thread- 

 like lines interspersed with minute pits, the radial arrangement of the 

 lines becoming less apparent towards genal angles. 



Inner regions of cheeks with straight posterior edge furnished with 

 narrow pleuro-occipital segment marked off bj' strong furrow, each 

 inner region thus forming a quarter of a circle, and connected in front 

 of the obtuse angles of circumferential region by narrow pre-glabellar 

 band of uniform width surrounding anterior end of glabella. Surface 

 of inner region of cheeks crossed diagonally by gently curved strong 

 line ('cheek-line') starting from middle of side of glabella and 

 running outwards and backwards to meet inner edge of circumferential 

 region a little in front of inner angle of genal prolongations, with 

 traces of small ' ocular ' tubercle on it at about the middle of the 

 cheek ; very minute punctse and extremely delicate striae present over 

 whole surface. 



Glabella oval, slightly angulated at front end, produced behind into 

 short blunt median spine, moderately convex, about two-thirds the 

 length of the head-shield ; small elongated median tubercle present, 



^ Salter, Mem. Gaol. Surv., vol. iii, p. 321, pi. xi a, figs. 9, 9a. 



