PSILOCEPHALUS. 175 



PsiLOCEPHALUS, 71610 f/enics, 1866. 



This has been long a MS. name, but only not yet made public owing to the great 

 delay in the pubHcation of vol. iii of the 'Memoirs of the Geol. Survey of North 

 Wales.' The genus is an inconspicuous one, and at first sight it looks as if it might be 

 placed with either Asaphus or Illanus. But the shape of the head, all but lobeless, as 

 well as the forward position of the small eye, easily distinguish it from Nileus, the group 

 of Asaphi which it most nearly resembles, while the eight grooved body-rings effectually 

 exclude it from Illcsnus. In the absence of the hypostome and labrum — essential 

 characters in the Asaphidds, it is impossible to characterise it fully. But I feel assured 

 it is a distinct genus. 



Oval, convex, especially the head, which may be described as inflated, with a faintly 

 marked out parallel-sided glabella, undefined in front ; no marginal furrow ; eyes very 

 forward and small ; head-angles obtuse ; body of eight rings, with grooved and facetted 

 pleurae ; it has an ungrooved tail, with a rather long distinct axis ; the metamorphosis is ■ 

 only partly known. 



Psilocephalus may be considered as forming the passage from Illcsnus to Asaphus, 

 by means of the abnormal subgenus Nileus of Dalman. 



PsiLOCEPHALUS iNNOTATUS, Salter. PI. XX, figs. 13 — 19. 



PsiLOCEPHALUS INNOTATUS, Salter. App., Ramsay, Geol. N. Wales; Mem. Geol. 



Surv., vol. iii, p. 315, pi. vi, figs. 9—12, 1866. 



p. late-ovatus, lavis, sescuncialis ; cvjus caput semiovatum caudd semicirculatd majus ; 

 sunt 8 pleurcB, fulcro ienus {ad tertias posito), plance dein decurvcB. Cauda lavissima, 

 axe prominulo. 



This neat and rather conspicuous form is the most abundant fossil in the Lower 

 Tremadoc beds, and usually found in company with the much rarer Niohe Homfrayi, 

 described at p. 143. We owe our best specimens to David Homfray, Esq., of 

 Portmadoc. 



The general shape is a rather broad blunt oval. It is scarcely an inch and a half, 

 seldom more than an inch, long, by about three quarters of an inch broad. The form 

 is very convex, especially in the head, which is but slightly lobed ; the tail more 

 strongly so, and the thorax strongly tri-lobed throughout. 



The head may be called hemispherical ; it is considerably longer than the tail, semi- 

 oval, blunt, very convex and smooth, not margined at all in front. The glabella is confused 



