30 SILURIAN TRILOBITES. 



found to differ when closely examined. Its glabella is much the same in shape, but rather 

 broader above ; the basal lobes not so strongly marked, especially in the cast ; the upper 

 furrows less defined, and making a less indentation on the sides ; the upper eye-lobe 

 broader and flatter. The cheeks, without being smaller, project less at the (blunt?) 

 angles and more at the sides, and the eye is a little further forward. 



But the most striking character, although it seems to be an unimportant one, is the 

 very distinct marginal furrow which subtends the eye closely in front, and leaves a strong, 

 broad, anterior margin to the cheek. It does not, however, quite meet the neck-furrow, 

 which rises towards it and runs nearly to the angle. 



This apparently neutral character is nearly the only one on which we can rely ; the 

 rest are proportional characters. But the species is a distinct one ; and although it is 

 almost too imperfect to name, the little Lower Silurian oasis in Cornwall, from whence it 

 comes, is of so much geological interest, that I venture to distinguish it. As the fossils 

 of that area are wholly dissimilar from those of other portions of the British Silurian 

 rocks, and only comparable with those of the " May " Sandstone of Normandy, it is 

 worth while to note the discrepancy by describing even the more obscure species. 



Calymene Arago, De Verneuil, and Homalonotus Brongniarti, Deslongschamps, accom- 

 pany it, and will be described under their appropriate genera. The same set of fossils is 

 found in the remarkable pebble-bed at Budleigh Salterton, in South Devon. See descrip- 

 tion of the next species. 



There is a certain resemblance to Dahnanites Phillipsi, Barrande, and even to Balm. 

 Haiolei, Barr., from the Lower Silurian rocks of Bohemia. But P. mimus is sufficiently 

 distinct from all. 



Localities. — Llandeilo Flags ? of Great Peraver, near St. Austell; in South Cornwall. 



P. (Acaste) incertus, Beslongschamps. PI. I, figs. 27, 28. 



Asaphus incertus, DesJongsch. Trans. Soc. Linn, de Calvados, toI. ii, p. 298, &c, 



pi. xx, fig. 5, 1825. 

 Phacops (Dalmannia) incertus, Rouault. Bulletin Soc. Geol. Fr., vol. viii, p. 371, 



1851. 



P. {Acaste) convexus, biuncialis, capite convexo, fronte angulato, caudd mucronatd. 

 Glabella antrorsum par urn dilatata, sulcis anticis hand profundis, posticis valde exaratis ; 

 mediano et postico arcuatis. Anguli genales brevispinosi. Cauda trigona lata ; axi 

 9-annulato, convexo, in mucronem longum crassum recurvum producto ; lateribus 6-sulcatis, 

 sidcis in terlin ea tis . 



About two inches long. We have only head and tail, the former convex, with a 

 glabella wider than the cheeks, and subparallel, or rather with the sides gently divergent 

 in front (not abruptly, as in P. socialis ; the front is bluntly pointed, as in that species). 



The glabella-furrows are well marked on the cast, and reach fully two thirds inwards, 

 towards the centre. The upper ones are oblique and sinuous ; the middle pair arched 



