34 TRILOBITES OF GIRVAN. 



Fixed cheeks with inner portion of anterior wing forming elongated rounded 

 semicylindrical roll, increasing in width posteriorly to nearly that of basal lobes of 

 glabella, descending steeply to occipital furrow. Ocular ridges narrow, rounded, 

 of uniform width, curved, following edge of fixed cheeks to eyes. Posterior wing 

 of fixed cheeks very narrow from back to front, horizontally extended laterally. 

 Facial sutures with anterior branches gently arched out, and slightly convergent 

 anteriorly ; posterior branches bent out at eyes, nearly at right angles to anterior 

 branches, and running obliquely back to cut posterior margin of head-shield 

 far out at base of genal spines. Meso-occipital furrow well marked, arched gently 

 forwards, marking off broad meso-occipital ring nearly one-third length of glabella 

 with strong median tubercle and pair of subcircular well-defined lateral lobes. 

 Pleuro-occipital ring of fixed cheek very narrow at axial furrows, expanding in 

 width outwards, convex, rounded, marked off by deep furrow. Surface of 

 cranidium ornamented with tubercles of two sizes, the larger ones fewer and 

 irregularly distributed. Free cheeks large, subtriangular, forming quadrant of 

 circle, convex, steeply arched down from eye ; border rather broad, rounded, 

 swollen; marginal furrow strong; margin furnished with series of 1-i — 16 

 equidistant, small, slightly recurved, short stout spines increasing in size 

 successively from facial suture to genal angle ; genal angle provided with stout 

 rounded tapering spine projecting backwards to about sixth or seventh thoracic 

 segment. Eyes small, circular, prominent, elevated, situated far back at about 

 one-fifth length of head-shield and behind middle of basal lobes of glabella. 

 Surface of free cheeks and of marginal and genal spines covered with almost 

 equidistant small tubercles of nearly uniform size. 



Thorax composed of ten segments. Axis narrow, cylindrical, decreasing very 

 slightly in width posteriorly, less than one-third total width of thorax ; very 

 convex and prominent. Axial rings with pair of distinct swollen lateral nodules 

 and ornamented with numerous small tubercles. 



Pleuras with inner three-fourths of their length straight and horizontally 

 extended ; outer fourth bent down rather suddenly, arched gently back, with the 

 extremity furnished with two unequal spines. Inner part of pleura divided 

 unequally by strong straight longitudinal furrow into narrower rounded anterior 

 band and broader more elevated posterior ridge ; anterior band ornamented with 

 single row of 6 — 8 equal and equidistant tubercles and produced into short 

 free recurved spine ; posterior ridge raised into prominent knob near outer 

 extremity and beyond it produced into long stout free spine strongly bent 

 back and down and overlapping anterior spine of next succeeding pleura ; posterior 

 ridge ornamented with marginal row of small tubercles and some scattered larger 

 ones ; general surface of both spines coarsely granulated. 



Pygidium transverse, broadly semicircular, marginally spinose, more than twice 

 as wide as long (excluding spines). Axis sub-cylindrical, narrow, less than one- 



