58 SILURIAN TRILOBITES. 



off, and somewhat overhanging the side-lobes. The eye does not reach the neck- 

 furrow. 



The tail is broad-triangular, wider than long ; the sides a little convex ; the apex 



short-mucronate ; the axis narrow, conical, ribbed by about sixteen rings ; the sides very 

 convex, with nine to ten arched simple ribs, scarcely at all interlined ; the margin narrow, 

 smooth. 



Locality. — May Hill Sandstone, "Long's Quarry, Damory Bridge, Charfield, and 

 Woodford Green, Tortworth;" (Decade 2, Geol. Surv., p. 7). 



Figs. 8, 9, show a variety with ribs slightly duplicated on the interior cast, but with a 

 short mucro. It is from Marloes Bay, Pembrokeshire (Mus. Pract. Geology). 



Larger form of the species. 



PL III, figs. 2, 3. 



The large caudal shields here figured clearly belong to a species distinct from P. caudatus. 

 They are one and a half inch long, and probably reached two inches in width when 

 perfect. The general form agrees well with that above described, and it has the numerous, 

 slightly curved ribs, scarcely duplicated at all, and with the interstices deeply concave. 

 They are very little bent at the extremities. Our fig. 2 is better in this respect than 

 fig. 3. The margin is equal and narrow all round. 



P. imbricatulus of Angelin has ten dichotomous side-ribs, and a very distinct margin ; 

 else it is not unlike our species. 



Locality. — Ludlow Rocks? Horseshoe Farm, Tortworth. It is possible there is a 

 boss of May Hill Sandstone at this place. 



Section — Cryph^us (p. 15). 



This subgenus alone would decide against the folly of classifying Trilobites by such 

 a character as the pattern of the tail. All the species known in Europe or North America 

 are allied to Odontochile, and follow naturally that subgenus. But in the southern 

 hemisphere we have compact forms allied to Acaste, which have the tail spinose. 



Gaspe, Lower Canada, to be identical with ours. The tail is a little more mucronate. The head agrees 

 nearly with our fragments, and will at all events give collectors a good idea of the probable shape. It is 

 remarkably triangular, even for this subgenus. 



