190 



SILURIAN TRILOBITES. 



front edge. It is as broad as long, slightly and very evenly convex all over, the broad 

 axal lobe being marked out in front by shallow depressions only, but no trace of axal 

 furrows, however short. The sides, too, show scarce a trace of the usual strong upper 

 groove — so conspicuous in most species. The greatest convexity is about the middle of 

 the caudal shield, and the edge is neither obtuse nor recurved. The front margin of the 

 tail is unusually sinuous. The front edge of the axis is greatly arched forwards ; the 

 fulcral point rectangular, and the outer angles widely truncate, so that from the fulcrum 

 the edge is almost vertically cut off. The fascia, narrow and convex at the outer angles, 

 becomes very wide and quite concave {i. e. convex in the cast) round the posterior 

 border, and is coarsely striate. 



Localities. — Llandovery Rocks. Abundant in the Kght-coloured sandstones of 

 Mullock, Girvan Water, Ayrshire (Mus. Pract. Geology and of Prof. Wyville Thomson). 

 Also the species is common at Haverfordwest, Pembrokeshire (Museum of the late 

 H. Wyatt-Edgell, and of Mr. Lightbody). May Hill Sandstone, Presteign : Builth : 

 and Pen-y-lan, Llandovery ; S. Wales. Tarannon Shale : Onny River, Shropshire 

 (Mus. Pract. Geol.). 



Subspecies II. 

 Ill^nus nexilis. pi. XXX, figs. 4, 5. 



This fossil appears to be intermediate between the ordinary //. Boicmanni and our 

 next species //. Thomsoni, which is evidently distinct. I cannot be so sure about the 

 present one, w^hich, while intermediate in form, possesses some characters not common 

 to either species. It has the aspect of //. Thomsoni, the even contour of tail, and arched 

 convex axis of the thorax, but not the abruptly reflexed pleurae or long truncate angles 

 to the tail of that species. The position of the eye and the convexity of the head are 

 like those of //. Bowmanni ; but it differs from that species — by having no trace of the 

 narrow neck-furrow so conspicuous in the Caradoc fossil ; and by having a thickened line 

 of apophyses below the axal furrows of the longer thorax, which thickened ridge produces 

 a broad and very definite axal furrow in the cast ; this is absent in Bowmanni. Again, 

 the tail is longer ; the front margin of its axal portion arched, not truncate ; the upper 

 lateral furrows all but obsolete ; the fulcrum closer in ; and the facet much more oblique, 

 thus truncating the angle very slightly. And the fascia, which first drew my attention 

 to this marked subspecies, differs entirely from that of //. Bowmanni, and is not quite 

 like //. Tliomsoni. I have only seen three specimens, all from the same locality, one of 

 which is a caudal shield, showing the considerable size of the species. 



Elliptical, 4 inches long, of which the large semioval tail forms nearly one half, while 

 the very convex (not gibbous) iiead is about one third longer than the strongly trilobed 

 thorax of nine rings. 



