34 BRITISH CAMBRIAN TRILOBITES. 



is somewhat shorter. It is larger than is usually the case in this species, but some 

 of the specimens mentioned by Ford exceed it in length. According to Matthew, 

 .If. sjpeciosiis has three pairs of broadly but distinctly impressed furrows on the 

 sides of the glabella, and also a pair of sharply cut occipital furrows, but on many 

 heads of adult examples all these furrows are quite obliterated. As the tail will no 

 doubt be found some time, I quote Ford's description : 



" Fygidium as long as the head and of nearly the same shape, but slightly 

 narrower, taking the extreme measurements, and more rapidly tapering ; grace- 

 fully curved in outline. Marginal rim distinct all round, widest anteriorly, dis- 

 tinctly raised or thickened at the edge. Axis conical, sometimes acutely so, long 

 and slender, extending very nearly to the margin, divided by faint cross furrows 

 directed slightly backward into eleven rings or segments. Side-lobes highly 

 convex and without furrows. The axis and side-lobes appear to overhang the 

 marginal rim at the posterior extremity, giving the border the appearance of being 

 only about half as wide behind as it is in front. 



" The entire surface is finely punctate. In one specimen of the pygidium, out 

 of a large number of perfect specimens examined, there appears to be a twelfth 

 ring in the axis." 



Horizon and Locality. — Cambrian : limestone beds 200 yards S. of Comley 

 Quarry, Shropshire. In America the species belongs to the Olenellus zone. 



3. Microdiscus sp. Plate III, fig. 8. 



A small tail found by Mr. E. S. Cobbold in the Olenellus band of Comley 

 Quarry is too imperfect to identify Avith certainty ; but as it is certainly different 

 from any species hitherto recognised in Britain, it is of sufficient interest to be 

 described and figured. It is an internal cast, without test. 



Tail semi-elliptical. Axis broad, conical, prominent, reaching nearly to the 

 posterior margin, distinctly segmented, four rings being visible on the portion pre- 

 served. Lateral lobes narrow, smooth, sloping downwards rather steeply from the 

 axis. Margin, or doublure, broad and concave (on the cast). 



Length 2*8 mm. ; width about o mm. 



It agrees very closely with the tail of M. helena, Walcott, 1 from which, indeed, 

 it appears to differ only in the width and concavity of the margin. The Comley 

 specimen, however, is an internal cast, and the apparent margin seems to be the 

 impression of the doublure. The difference, therefore, is probably apparent rather 

 than real. 



Horizon and Locality. — Olenellus limestone: Comley Quarry, Shropshire. 



1 Tenth Ann. Rep. U. S. Geol. Surv. (1890), p. G32, pi. lxxxi, figs. 1, 1«. 



