296 Frederick Chapmen^ : 



evenly rounded. Lateral border rounded, and prolonged pos- 

 teriorly into a short genal process. Tliorax moderately convex 

 alonj^ the median line. Axal furrows well marked. Body rings 

 ten, short, of even width, gently curved on the axis, smooth, 

 with granulate or striate margins. Pleura moderately narrow ; 

 about half the width of the axal segments. Fulcrum somewhat 

 sharply bent ; distal portion of pleura pointed at the extremi- 

 ties. Pygidium long, semiovate, convex in median area ; sur- 

 rounded by a moderately broad fascia of even width, finely 

 striate. Axis continued some way into the tail, defined by two 

 rapidly converging furrows, the extent of which is not clear in 

 the specimen described. 



Dimensions. — Approximate. Specimen slightly and obliquely 

 crushed. Total length, 37.25 mm. ; length of cephalon, 14 mm 

 width of cephalon, (cir.) 16 mm. ; length of thorax, 7.25 mm 

 greatest width of axis, lO mm. ; width of pleura (cir.) 3.25 mm 

 length of pygidium, 16 mm. ; width of fascia, 2.25 mm. 



Affinities. — This handsome and distinct species is a tyi>ical 

 Illaenus, in having ten body rings and a moderately narrow 

 axis. One of the nearest related species is /. davisii Salter^ 

 ( = /. crassicauda, Sharpe), which is proportionally wider than 

 the present species, has a broader and more tumid cephalon, 

 blunt, genal angles, longer lateral pleura, and a rounder 

 and shorter pygidium. Moreover the ornamentation of the 

 glabella is a distinguishing feature in the Victorian specimen. 

 It is of especial interest to note that the British /. davisii is 

 found in tlie more argillaceous parts of the Bala limestone (upper 

 ordovician) in Wales ; and it also occurs in strata of equivalent 

 age in Scotland. 



Other simulant forms, both from the llandovery series (base 

 of Silurian proper) are Illaenus maeeallunii, Salter"-, and /. 

 nexilis, Salter". The first-named, however, is a Bu?nastus, with 

 a typically broader l)ody and well-developed eyes ; whilst the 

 second belongs to the section Dysplanus, with only nine body 



1 Mem. Geol. Siirv. United Kiiij^dom ; Brit. Foss., Dec. ii., 1849, pi. ii. 



2 Mon. Sil. Trilobites (Pal. Soc. Mou., vol. .\x.), 1867, p. '210, pi. xwii., fiJ,^ 1; pi. 

 XXX. figs. 2, ?,. 



3 Ibid., p. 100, pi. \x\., ti^'.s. 4, 5. 



