652 THE LOWER SILURIAN PERIOD. 



Occipital ring strongly arched upward, and separated from glabella 

 by a well-marked groove ; middle of posterior margin produced 

 backwards in a short conical spine. Fixed cheeks highest along 

 dorsal furrow, towards which they pressed abrupt round slopes, 

 while their general surface slopes gently and quite evenly towards 

 front or sutures. The dorsal furrows are confluent in front with the 

 flat margin, so that the cheek lobes do not meet in front. They are 

 highest along the straight dorsal furrows, but where they bend to go 

 round the anterior extremity of glabella, the cheek-lobes narrowing 

 and curving towards each other, gradually sink away and disappear 

 in the front flattened space. 



The ocular lobes are very well developed, forming sub- semicircular 

 lappet-like lobes, curved strongly upwards, and situated about opposite 

 to the centre of the head. An ocular ridge, low and rounded, but very 

 prominent, runs from anterior margin of ocular lobes, with a curve 

 almost parallel with front margin of shield, but slightly divergent 

 from it to the dorsal furrow, which it gains at a point considerably 

 back of front of glabella, and where the straight part of the dorsal 

 furrow bends to go round the front. Posterior limb short and 

 broadly rounded. Post-marginal furrows less deep than dorsal, 

 wider ; marginal fold narrow and moderately prominent ; shield 

 strongly arched transversely ; surface smooth. 



Cephalic shields without fixed cheeks, only part preserved. Rather 

 uncommon in the Trilobite shales of Ratcliffe's Millstream. — N. B. 

 Survey, 1864, and J. W. Hartt. 



Conocephalites tener, Hartt, MS. Minute, glabella ovate-conical, 

 truncate at base, rounded in front, where it is about half as wide as at 

 occipital furrow ; slightly contracted behind ; length about equal to 

 width at occipital furrow ; strongly depressed convex, mOre elevated 

 at base than at front, and higher also than fixed cheeks ; aspect varies 

 with state of preservation of specimens ; arcuate, rounded, convex, or 

 concave ; the middle seems to be inclined to project back slightly 

 over the occipital furrow ; slopes abruptly to occipital furrow, which 

 is moderately deep, wide, and narrowed, and slightly inclined forward 

 at the ends, where it terminates abruptly ; bounding groove deeper 

 than other grooves in head ; occipital ring projecting backward 

 bodily beyond higher margin, with the axis of its fold inclined more 

 or less backward, and produced in the middle into a short conical 

 backward inclined spine ; anterior limb regularly arched as if the 

 outlines of the complete head were semi-circular. 



Fixed cheeks anterior border broad, flat-concave, rising more or 

 less abruptly to a sharp, thin, marginal fold ; width between anterior 



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