DIKELOCEPHALUS DISCOIDALIS. 115 



margin ; near the axis the lateral lobes are almost level, but farther out and 

 posteriorly they bend down to the broad, flat margin; the furrows on the lateral 

 lobes curve backwards, the anterior furrows following the curve of the anterior 

 border, and the later ones being bent still more strongly backwards. 



Dikelocephalus is characteristically an American genus, and, indeed, in his 

 revision Walcott does not allow a single species outside America ; but he leaves 

 some doubtful, and he refers the Scandinavian form Dikelocephalus ? leptasnarum, 

 Wiman, to the closely allied genus Sauhia, Walcott. Dikelocephalus celticus, Salter, 

 however, shows all the characters of a true Dikelocephalus tail, the only special 

 feature being the slight emargination posteriorly; and in D. discoid '-a lis, Salter, the 

 facial suture takes the characteristic course for Dikelocephalus, the eye has the 

 proper position and the glabella the right shape, but the specimens are too 

 imperfect to show clearly whether the posterior glabellar furrows were continuous 

 across. 



1. Dikelocephalus discoidalis, Salter. Plate XIV, figs. 2 — 7. 



18(36. Dikelocephalus (Centropleura) ! discoidalis, Salter, Mem. Geol. Surv., vol. iii, p. 304, pi. v, 

 figs. 18, 19. 



Head forming an arc of a circle, with the genal angles produced into spines. 

 Glabella less than two-thirds the length of the head, quadrate, narrowing slightly 

 forwards, truncate or even slightly emarginate in front, with three pairs of 

 glabellar furrows and a strongly marked neck-furrow. Eyes rather large, 

 crescentic, placed far back, very near to the occipital furrow and to the glabella. 

 Facial sutures meeting at an obtuse angle in the median line a considerable 

 distance in front of the glabella, thence running obliquely outwards and back- 

 wards, and then curving strongly inwards and backwards to the eye, behind which 

 they turn outwards almost at right angles to the axis and cut the posterior margin 

 at a distance from the axial furrow about equal to the width of the glabella at its 

 base. Fixed cheeks forming in front of the glabella a broad, flat expansion, which 

 on the internal cast shows a fine lineation ; very narrow at the eye, and behind 

 the eye extending outwards as a narrow wing marked by a strong occipital furrow. 

 Free cheeks wide, gently arched, bordered by a rather narrow margin of very 

 uniform width, which projects in front of the cranidium; genal angles produced 

 into rather slender spines of no great length. 



Dimensions. — The head, before distortion, was probably 15 — 20 mm. long, and 

 more than twice that breadth. 



D. discoidalis is probably the head of D. celticus. It occurs at the same horizon 

 and in the same locality, and the sizes are not discordant. It should be noted 

 that the specimens of the free cheek figured on Plate XIV, figs. l>, 7, do not come 



