6 



BRITISH FOSSILS. 



eighth segment is only seen in the exterior crust (fig. 4, a); the inte- 

 rior cast of the same segment (see figs. 3 and 4) shows nothing of it. 

 The tail is oblong, the posterior edge cut into four strong teeth, the 

 two outermost shorter than the others. The axis two-ringed, as in 

 the other species. 



British Locality and Geological Position. — Lower Silurian; 

 Desertcreat parish, Tyrone (fig. 3.), in fine micaceous sandy schists ; 

 Tramore, Waterford (fig. 4), in arenaceous slate. 



Variations. — These three supposed species have purposely been 

 described and figured together, in order to show how very trivial 

 the variations are between them, except, of course, in the remark- 

 able appendages to which the specific names refer. R. laterispinifer 

 has the general axis somewhat broader than R. Colhii; and this 

 again than B. dorsospinifer. The two first-named species, indeed, 

 agree very nearly in its proportions, as it tapers in the body seg- 

 ments rapidly fi:om front to back. In the last form, which is more 

 elongate and narrow than either, this tapering is not nearly so 

 rapid. But the general shape, configuration of the glabella and 

 cheeks, the extent, size, and position of the eyes, the broad axis of 

 the body rings, and the short hatchet-shaped pleurse, are the same 

 in all ; each has the remarkable produced fulcral point, placed close 

 to the axis — and the tail, as far as it is preserved in each species, 

 shows no difference in character. The surface, too, appears granulose 

 in all, and the posterior edge of the body segments is serrated by a 

 projecting row of tubercles. 



The only striking peculiarities reside in the appendages, the first 

 having neither lateral or dorsal spines ; the second having the seventh 

 pair of pleurse produced into spinous points ; and the last, together 

 with a more elongate general form, is furnished on the eighth seg- 

 ment of the axis with a strong dorsal spine. 



Sex. — How far these variations may be regarded as differences of 

 sex, is a point worthy of consideration. It is well known that a 

 narrower form, and additional ornament frequently characterizes the 

 male of other Crustacea. In the former Decade we have endea- 

 voured to apply this to the observed differences between certain 

 species of Phacops, — and in the present one to Cyphaspis. 



M. de Barrande has, indeed, shown that there generally exists 

 among the Bohemian Trilobites a broad and narrow form of each 

 species ; and he has particularly noticed this in the case of A cidaspis 

 {Odordopleura), and considered the narrov/er form that of the male. 



