OLENELLUS ZOKE IN" THE :NrOETH-WEST HIGHLANDS. 237 



margined on the front and sides by a raised border, rounded oif 

 outside and separated from the cheeks and frontal area by a 

 depression. This border is narrow, but varies in breadth in different 

 individuals, being nearly twice as wide in some as it is in others. 

 In all it is widest nearest the postero-lateral angles and gradually 

 diminishes in front towards the middle line, being about one third 

 narrower in front than behind. At each of the postero-lateral 

 angles the carapace is continued into a long strong spine, directed 

 backwards and slightly outwards. The spine also varies in different 

 individuals, being longest and strongest in the most elongated forms. 

 On an average the spine measures about two thirds of the length of 

 the carapace. 



The posterior margin is concave and divided into three areas, viz. 

 a central one coinciding with the boundary of the occiput, and two 

 side wings. Each of these latter has a thickened border, which 

 begins narrow at the edge of the occipital ring, widens out 

 gradually for | of its extent, and then suddenly tapers off just 

 before reaching the cheek-spine, from which it is cut off by a wide 

 and shallow notch. IN"© tubercle or rudiment of a spine is observable 

 at the angle where the band bends into the notch. 



The cheeks and the frontal area are continuous, rising gently and 

 evenly from the depression bordering the marginal fold ; but since 

 the fossils are preserved in shale and are in consequence somewhat 

 compressed, there is no means of finding out from them the amount 

 of original convexity. The glabella and eye-lobes rise steeply out 

 of the combined genal and frontal area, the external or visual wall 

 of the eye-lobes being almost vertical. 



The glabella is club-shaped, rounded in front, and widest where 

 the eye-lobes merge into it just in front of the foremost furrow. 

 From this point it tapers very gradually backwards ; it is crossed 

 by three furrows, each of which begins at the outer margins, 

 passes forward, and suddenly bending round upon itself, passes 

 inwards and backwards to meet its other half in the middle line. 

 Each furrow is deepest at the two angles and shallowest at the 

 middle line. A fourth, the occipital furrow, crosses from side 

 to side with a straighter course than the others, though it is 

 still bent backwards a little in the middle line. As a consequence 

 of this arrangement of furrows the anterior portion of the carapace 

 is cordate ; the three succeeding lobes are M-shaped, and the 

 occipital ring, which is large, is deeper at the sides than in the 

 middle line. The occipital ring bears a short rudimentary spine or 

 tubercle in the middle line, near its posterior margin. 



The eye-lobes are crescent-shaped and are more than -i- the 

 length of the carapace. Anteriorly they merge with the glabella 

 just in front of its first furrow. Thence they extend backwards 

 and slightly outwards, and end at a point a little behind the 

 occipital furrow and a considerable distance from the edges of the 

 occipital ring. 



The exterior margin of the orbit is steep, and from its crest the 

 test sinks more gently inwards to the general level of the head. 



