SPH^EROPHTHALMUS ALATTS. U 



Head quadrate, emarginate in front, with rounded sides, from the middle of 

 which the short genal spines spring abruptly. Glabella quadrate, longer than 

 wide, slightly narrowing forwards, truncate in front, just reaching the marginal 

 furrow. Anterior glabellar furrows obsolete, the posterior pair well defined, 

 meeting across the glabella, but rather shallower in the middle than at the sides ; 

 neck-furrow strong ; neck-segment occasionally showing traces of a median tubercle 

 or spine. Eyes large, round, placed very near to the posterior margin and at a 

 distance from the glabella equal to rather more than half its width; a very oblique 

 ocular ridge visible in some specimens, especially in external casts, but usually 

 absent. Facial suture running in a gently sigmoid curve from the anterior to the 

 posterior margin, its general direction being approximately parallel to the axis. 

 Fixed cheeks narrow, convex, very little expanded behind the eye ; free cheeks 

 with the outer border forming a large arc of a circle, margin rather broad and flat. 

 Genal spines short and slender, springing from the middle of the outer margin, 

 bent backwards a short distance from their origin. 



Thorax of nine segments, widest about the third segment, much contracted 

 behind. Axis broad, forming more than one-third of the total width, rather convex, 

 with a median tubercle on each segment. Pleuras short, rapidly decreasing in length 

 posteriorly, with broad oblique grooves, bent downwards at one-third of the way 

 out from the axial furrow ; extremities of the first two or three bluntly pointed, of 

 the later segments not clearly seen. 



Tail small, triangular, width rather less than twice the length, margin entire. 

 Axis conical, broad, forming the greater part of the tail, consisting of three 

 segments including the terminal portion, ending in a fairly sharp point, not quite 

 reaching the posterior margin, lateral lobes narrow. 



Dimension*. — An average cranidium is about 2 — 2'5 mm. lung and 2'2 — 3 mm. 

 wide ; the thorax and tail together about 3*5 — 5 mm. long, 3 — 4-nim. wide. 



It should be observed that on internal casts the ocular ridges are seldom seen 

 and the median tubercles on the axis of the thorax are often indistinct ; but their 

 apparent absence seems to be due merely to the mode of preservation. According 

 to Salter there are only seven thoracic segments ; but he seems to have included a 

 part of the thorax with the tail, for he describes the latter as possessing three 

 spines on each side. Probably in his specimens, as is often the case, the hinder 

 end of the body was badly preserved. He also remarks that the free cheeks (so 

 far as he could see) show no trace of the curved spine characteristic of the genus, 

 but evidently he was not quite satisfied on this point. According to Belt 1 the tail 

 is minute and furnished with a long terminal spine ; but there is no indication of 

 such a spine in any of the specimens that I have examined. 



The specimen shown in Plate VIII, fig. 6, closely resembles Linnarsson's figure 



1 Geol. Mag., vol. v (1868), p. 10. 



