134 CAMBRIAN TRILOBITES. 



The tail, nearly semicircular in outline, is more than twice as wide as long. The 

 axis broad, quickly tapering, reaching less than four fifths the length of the tail ; depressed, 

 and with (apparently, for the extreme tip is lost) a blunt termination. It consists of 

 eight strong rings, and a semicircular terminal portion. The broad side-lobes are 

 grooved by eight strong primary furrows, interlined from near their origin by furrows 

 as strong as themselves, but which commence very abruptly at a short distance from 

 the axis, and radiate straight out, not bent (as are the primary furrows) in a line corre- 

 sponding to the fulcrum of the pleurse. A broad band of very oblique and wavy striae 

 ornaments the border of the tail above, and beneath it the concentric striated fascia 

 (fig. 12) has the striations wide apart. 



The labrum (fig. 13) is pointed abruptly, and seems to have two pairs of concentric 

 furrows, but it is very imperfect in our specimen (fig. 13) from the Tremadoc Rocks. 



In fig. 10, which is from the Arenig group, the labrum is more perfect. It is as 

 broad as long, and subquadrate in outline, the front much arched ; the base of attach- 

 ment scarcely wider than the broad- winged sides ; the notch above the short auricle 

 somewhat acute. The concentric furrows are strong and double, the central part enclosed 

 by them roundish and convex, and the apex (broken off) is connected with this central 

 part by a raised space, and this elevation is sharply divided from the flat wings by 

 longitudinal furrows. 



The large size and round form of this fine species much recals that 0. JDesmaresti, 

 Brongn. (the 0. Brongniarti of Rouault), and well figured by the latter authority in the 

 ' Bull. See. Geol. France,' vol. vi, 2nd ed., pi. i. From that species the less width of the 

 axis, the straight, not curved, pleurae, grooved nearly to their ends, distinguishes our rare 

 fossil. From 0. Edwardsii (pi. ii, fig. 1, of the same work) it is distinguished by the fewer 

 joints of the tail, and the less number of lateral furrows, all of which are duplicated. The 

 pleurae, too, are curved in 0. Edwardsii, Rou., which in many respects closely resembles 

 0. Besmaresti. I do not know any species with which ours need be compared, except 

 the following one, which at one time I thought identical ('Siluria,' 1. c, p. 53). 



Localities. — Lower and Upper Tremadoc Rocks. Lower Tremadoc. — Carnarvon 

 Road, 1^ miles W. of Tremadoc (Mr. Homfray). Upper Tremadoc. — North face of 

 Garth Hill, at the mouth of the Traeth Bach, Merionethshire, opposite Portmadoc — in 

 company with Angelina and many other fossils (Mus. Pract. Geology). Also Portmadoc, 

 quarries in the town. The labrum (fig. 12) is from thence, and is in Mr. Ash's cabinet. 

 Arenig group. — North-east corner of Whitesand Bay, St. David's, Pembrokeshire, figs. 

 9, 10 (in Mr. Lee's cabinet). 



