ILLtENOPSIS. 



213 



most perfect form Bumastus, in which the characters of the group may be said to 

 culminate. As only one species is known, it is desirable to unite the generic and 

 specific characters in description. 



Ill^nopsis Thomsoni, Salter. PI. XX, fig. 1. 



Ill^nopsis Thomsoni, Salter. Mem. Geol. Survey, vol. iii, pi. II b, figs. 1,2, 

 pp. 2.56, 316 (not of the May Hill, or Upper Llandovery 

 list in p. 360, which is a misprint for Illanus Thom- 

 soni), 1866. 



II. modicus, vix 2 uncias longus, 1^ latus, ovatus ; capite convexo obtuso, sulcis 

 conspicuis; ocuUs anticis. Caput semicirculatum, convexissimum. Glabella urceolata, genis 

 multo latior, sulcis axalibus profundis superne divaricatis et ad marginem deductis. Oculi 

 parvi antici, marginales, angulo glabella insidentes. Pleura cuspidata, sulcata. Cauda 

 {lime speciei haud dubie referta), semicirculata, axe longo distincto ; sulco svperno conspicuo. 

 Fascia angusta, axem attingens. 



I have shortly described this form as "ovate, the head very convex, with widely 

 divergent axal furrows reaching to the front ; technically, the axal furrows are complete. 

 Fixed cheeks large; free cheeks small and anterior; the eye placed close to the front 

 margin ; facial suture ending on the outer margin ; pleurae pointed, grooved." [Tail 

 margined, and with complete axis.] See ' Mem. Geol. Surv.,' vol. iii, 1866. 



Head semicircular, very convex ; rather more so in front, but not gibbous. The 

 glabella, much wider than the cheeks, is marked out by two strong deep furrows, parallel 

 below, but curving out so much above as to give a sbeaf-like look to the glabella. The 

 axal furrows continue quite to the front edge, as strong as at the base ; and at the front 

 they overhang the small eye, which is close to them. The eye-lobe is not raised. The 

 eye is placed in a rather deep notch, and so close to. the front margin that the free cheek 

 (separable, certainly, for it is absent) must have been quite minute and the facial suture 

 all but marginal. Below, the wide fixed cheeks have rather a strong neck-furrow, 

 which does not, however, extend beneath the glabella at all. 



Of the thorax we have only the two upper pleurae, but these are characteristic. Much 

 shorter than the width of the cheek, they present a triangular-pointed outline, the tips 

 being acute and a little curved forwards ; a strong oblique groove traverses most part of 

 their length. 



The tail found with this head can hardly belong to any other species. It is semi- 

 circular, with very slightly truncated front angles ; and has the axis distinct for fully 

 four-fifths the length of the tail, and marked sharply out all round (another reason for 

 regarding Illanopsis as a genus intermediate between Blanus and Asaphus or its allies). 

 The axis is narrow, rather more than a fourth the whole width of the tail, and with the 

 first ring strongly marked out, at least in the cast. The sides are smooth, except the 



