2 BRITISH FOSSILS. 



teriorly, not so wide as the pleurae in the anterior rings, but in the 

 hinder ones wider than the pleurae. Tail semicircular, truncate, the 

 axis convex with one rib above, and about equal in width to the sides, 

 which also have a single faint rib. Margin entire, slightly raised. But 

 four specimens have been found, and all more or less distorted. 



Affinities. From O. spinulosus, with which the head has considerable 

 resemblance, the entire truncate one-ribbed tail readily distinguishes it. 

 O, gibbosus, besides having a triangular, many-ribbed tail, has the gla- 

 bella not reaching nearly to the front margin, and the eye is placed at a 

 distance from the glabella nearly equal to the diameter of the latter. 

 In O. micrurus it is not above half the diameter distant. O. alatus, a 

 small species, named by Dr. Boeck, has the glabella extended forwards, 

 but it is very narrow and convex, and the eye is distant. The Olenus 

 gibbosus (Corda), which, in shape of tail, is very different from Swedish 

 specimens and Wahlenberg's description, differs from ours in the wide 

 and ribbed tail, as well as in the general shape, which is oval, not at- 

 tenuated ; besides other points. We know of no other species with 

 which it can be confounded. 



British Localities and Geological Position. LOWEST LLANDEILO 

 FLAGS (Lingula beds), Trawsfynydd, Merionethshire ; Cwm-y-Swm 

 mine, Dolgelly, ditto ; Marchllyn Mawr, near Llanberis, Caernarvon- 

 shire. 



EXPLANATION OF PLATE IX. 



Fig. 1. Olenus micrurus. Trawsfynydd. 



Fig. 2. Same dissected, a, central lobe of the head,* including 5, the glabella, and c, 

 fixed cheeks; d, the mucronate wings; e, thorax segments, the obscure fulcral points 

 are at//; tail, g, with two-jointed axis, and single lateral furrow. 



Fig. 3. Small specimen, Cwm-y-Swm mine, Dolgelly. 



Note on the Genus. 



The group of OlenidcE (Emmerich) includes genera which differ from each other in the 

 course of the facial suture ; in Paradoxides and Olenus its termination is on the posterior 

 margin, and the wings or free cheeks are produced into spines. In Pelttira (P. scara- 

 bceoides) and Triarthrus, it terminates in the posterior angle, or even a little above it. on 

 the external margin, and the wings are very small, and by no means produced into spines. 



Paradoxides differs from Olenus in the large and clavate, not bell-shaped, glabella ; in 

 possessing 16 to 20 body rings, and a minute tail, with its axis only articulate. 



Olenus is divisible into two sections, if these do not merit the name of genera. 1. The 

 group of true Olenus, to which our species belongs, having 14 body rings, and a tail with 

 entire margin. 2. O. spinulosus, having fewer, about 12 body rings, and a spinose or 

 laciniate tail ; for the latter species, the sub-generic name Parabolina, in allusion to the 

 shape of the glabella, might be appropriate. 



June, 1849. J. W. SALTER. 



* Corda has proposed to call all this " glabella," and to distinguish it into central and 

 lateral lobes. 



