4 PKOCEEDINGS OF THE NATIONAL MUSEUM vol. 76 



usually considerably wider than long, large as compared to the thorax 

 and pygidium, with very large, often bulbous, eyes and narrow rim. 

 Glabella well defined, usually strongly convex, tapering more or less 

 forward, rarely semielliptical to subovate, without lateral furrows 

 but in some species with a dimple near the middle of the lateral slopes 

 and more rarely with smooth ovately outlined spaces that probably 

 rej)resent the posterior and second glabellar lobes of other trilobites. 

 Occipital ring well developed, often crescentic, usually with a me- 

 dian spine that varies greatly in length and strength with the species. 

 Fixed cheeks rather narrow posteriorly, increasing in width ante- 

 riorly so that the greatest width is more or less in front of the middle 

 and the outline varies from crescentic to rounded-triangular 5 outer 

 })order made by a concave palpebral band that flattens along the 

 anterior side beyond the eye; area between this band and the dorsal 

 furrow more or less strongly convex, usually rising into a curved 

 ridge. Posterior portion of facial suture directed obliquely out- 

 ward, backward, and downward around the very small posterior 

 limb to the point only a short distance from the dorsal furrow at 

 which it cuts the posterior margin. Anterior part of facial suture 

 beyond the eye closely following the anterior edge of the cephalon 

 to a small projection of the rim which it cuts to reach the edge. 

 Between these slightly projecting points the anterior rim is emargi- 

 nated and the excavation divided unequally into three concave parts 

 by two hollow spines that are highly characteristic. These spines 

 sometimes project directly forward, but more commonly they curve 

 gradually or more abruptly downward. The cavities on their outer 

 sides are smaller than the median emargination and as a rule partly 

 bottomed by shell. Free cheeks consist mainly of very large bulbous 

 or somewhat crescentiform eyes separated on the outer side by a 

 narrow deep groove from a narrow wirelike rim that broadens more 

 or less near or somewhat in front of the middle of its posterior half 

 to give sufficient lodgment for the base of a genal spine. This 

 spine varies greatly in size and direction. Rarely it is weak and 

 short, oftener strong, long and curved and directed outward at 

 varying angles; and in one case it rises directly upward from the 

 rim and curves over the eye. Occasionally a smaller though other- 

 wise similar spine occurs a short distance behind the genal spine. 

 The eyes, as said, are very large and more or less strongly convex, 

 and numerously facetted as in Aeglina^ and in some cases at least 

 must have been set on the head so that in a dorsal view of the animal 

 the outer rim of the cheek would be almost .covered by the periphery 

 of the eye. In such cases the genal spines are very small or want- 

 ing, as in T. inysticensis, or they are turned up beside and doubtless 

 beyond the top of the eye, as in T. piistulatus. Facettes on eyes ar- 



