732 DR R. H. TRAQUAIR ON 



Of Uncertain Sub-Class. 



Order ARTHRODIRA. 



Family Coccosteid^e. 



Genus Coccosteus, Agassiz. 



Coccosteus angustus, Traquair. PL VI. figs. 1 and 2. 



Coccosteus angustus, Traquair, Nature, vol. liv., 1896, p. 263; Proc. Brit. Assoc, Belfast, 1902, p. 263. 



Specific Characters. — Ventral cuirass narrow ; median dorsal plate, with a median 

 crest on the second fourth of its length. 



Description. — Only one specimen has occurred, which has been worked from both 

 sides by Mr Sturtz's assistants. 



The ventral side, PL VI. fig. 1, is the more perfect, and shows, besides the ventral 

 cuirass, the displaced suborbital and median dorsal plates. The ventral cuirass shows 

 the unmistakable contour of that of Coccosteus, but the lines of demarcation between 

 the individual plates cannot be made out, although a good many cracks, the result of 

 crushing, are very distinctly marked. The position of the anterior median ventral plate 

 is covered by a growth of pyrites, and the posterior one is also indistinguishable. 



The anterior ventro-laterals are of the form usual in Coccosteus, and, as in C. 

 decipiens, the tuberculation of the surface is fine in the centre of the plate, coarse along 

 the anterior, inner, and posterior margins. Along the anterior external margin is seen 

 the short, blunt, pectoral spine, which projects just a little outwards beyond the external 

 angle, as it does in C. decipiens. 



The part of the cuirass occupied by the posterior ventro-laterals is narrow, hence the 

 specific name ; but, as in C. decipiens, the left one overlaps the right. 



Just in front of the cuirass is the displaced left maxilla or suborbital plate, showing 

 the usual sensory groove bifurcating below into an anterior and posterior branch. 



Then, lying close to the outer margin of the right posterior ventro-lateral is the 

 median dorsal, torn from its place and turned upside down, so that we see its upper or 

 external surface on the same side of the slab which displays the ventral plates. Its 

 form is narrow-oblong, truncate in front, faintly pointed behind ; its length is If and 

 its breadth ^-| inch. In the middle line and in the second fourth of its length is a 

 median crest, which gradually rises anteriorly, suddenly ceases posteriorly, and in the 

 present specimen is bent over by crushing to the right side. The surface of the entire 

 plate is ornamented by stellate tubercles, which toward the posterior extremity are 

 arranged in concentric lines. 



In fig. 2 of Plate VI. we have a view of the same specimen worked out from the 

 dorsal side, whereby a certain amount is added to our knowledge of the species. In 

 front is seen a portion of the upper surface of the cranial shield, showing very distinctly 



