﻿Vol. 66.] THE CAMBRIAN ROCKS OF COMLEY. 29> 



of the glabella ; the fixed cheeks appear to be rather narrower ; the- 

 hollow parallel with the eye-lobe and the ocular ridge is more 

 distinct ; the frontal limb is thickened and curved downwards to 

 the front margin, with an extension forwards equal to one-third of 

 the length of the glabella, and, though flattened in front of this 

 latter feature, does not show any decided hollow or marginal rim. 



MlCMACCA ELLIPSOCEPHALOIDES, Var. STRENT7ELLOIDES. (PI. VIII,. 



figs. 3 & 6.) 



Two cranidia [J 12904 & J 12902J are somewhat wider across- 

 the base and narrower in front between the sutures than is the- 

 case in the type-specimens, and in this respect bear a resemblance 

 to Jgraulos (Strenuella). The frontal limb stands out horizontally 

 to a greater distance, and has the form of the toe of a slipper 

 when seen from the side; and in one specimen [J 12902] the 

 glabella is narrower in front than behind. In both cases the 

 occipital ring is incomplete, so that it is impossible to say whether 

 it bore a spine or a tubercle. This last specimen also show r s traces 

 of at least seven thoracic segments in place : these are so much 

 damaged, that it can only be said that the number is incomplete ; 

 that they have a very convex axis ; that the pleurae stand out hori- 

 zontally for a distance equal to about two-thirds of the width of 

 the axis, and are there bent downwards at a steep angle to their 

 terminations, which are also bent backwards and probably ended in 

 sharp points. The traces of these points in the rock are concave 

 throughout, implying the existence of an extended doublure. 



On the same piece of rock there is a w r ell-preserved fixed cheek,, 

 showing the sutures, and approaching more nearly to the type 

 (PI. YIII, fig. 4). 



MlCMACCA ELLIPSOCEPHALOIDES,. Var. SENIOR. (PI. VIII, fig. 2.) 



A cranidium of larger dimensions (length = 13 mm.) is represented 

 in the specimen [J 12901] shown in fig. 2. There is a robustness- 

 about it which suggests that it is an older individual. It departs 

 from the type, principally in the greater proportionate width across 

 the posterior branches of the facial sutures ; in the occipital furrow 

 being quite lost in the middle of its length, but wide and deep at 

 the sides ; and in the very thick but shortened frontal limb, with 

 a decided hollow between it and the glabella. The occipital ring 

 seems to have been devoid of spine or tubercle. 



The divergences between these four forms are, when considered 

 together, too slight to warrant a separation into diverse species, and 

 I am doubtful wiiether they have any permanent varietal importance. 

 It seems more probable that the species was in a state of flux and 

 that it was not until later, if at all, that it crystallized out into forms 

 with well-marked permanent differences. 



Locality and horizon. — Comley, from the Olenellu's Lime- 

 stone of the quarry and of the excavation 200 yards south of it. 



