TRILOBITES OF THE UPPER SILURIAN ROCKS. 



657 



smooth. (See PL 7» f» 3 a and b.) The facial suture (linea facialis) traverses the oculine pro- 

 tuberances, separates the upper portion of them from the eye, and p)(isses under the margin. 

 (See PL 7 bis. f. 3 b*.) Body with no true longitudinal furrows ; central lobe only just per- 

 ceptible by very slight depressions in the body only. Ribs 10, those of the lateral lobes ter- 

 minating in recurved blunt ends. Caudal portion round and smooth without a trace of trilo- 

 bation. (See PL 14. f. 70 Surface of the whole animal {testa) covered by extremely thin, 

 apparently imbricated lamellce, the edges waved or vermiform, the intermediate spaces studded 

 with minute dots. (See magnified portions of the eye and head, PL 7 bis. f . 3 c and d.) Where 

 exposed to friction during the life of the animal these markings are removed. 



This remarkable crustacean has been hitherto known in England as the Barr trilobite, having 

 been found at the Hay Head lime works, near the village and beacon of Barr in Staffordshire. 

 A very large specimen of it, 5 inches long by 3^ inches wide, has been lithographed at Bir- 

 mingham, and the species has been figured under the English name above stated by Mr. F„ 

 Jukes and Mr. J. Sowerby, in Loudon's Mag. Nat. Hist. vol. ii. p. 41., accompanied by a 

 wood-cut. 



I formerly supposed that, from its equally balanced extremities, this crustacean belonged to 

 the genus Isotelus (Dekay), p. 215., but since the earlier chapters of this work were printed, 

 I perceived that the English fossil was excluded from the American genus by the absence of 

 distinct trilobation in the body, and of any trace of such divisions in the caudal portion, by 

 having 10 instead of 8 ribs, and by other essential differences. 



I next observed, that although agreeing with the Illcenus of Dalman in the number of ribs, 

 that genus differed from our fossil in having the head plain and without protuberances, eyes 

 strongly reticulated, body distinctly divided into 3 lobes, the longitudinal depression being ex- 

 tended into the head and caudal portion, and the lateral ends of the ribs sharp. Though our 

 fossil approaches to Nileus Armadillo, Dalman, in the almost entire absence of longitudinal 

 trilobation, that genus is very distinct from ours, by having eight ribs only, which terminate in 

 points, a plain head, and very large reticulated eyes. Again, I was once disposed to think that 

 the mutilated specimen, without a "pygidium " figured by Eichwald as Cryptonymus Rosen- 

 bergii, De Tril. Obs. Casan, PL 3. f. 3 a and b., might be identical with our species, but on 

 receiving (while these pages were printing) the rare work of Pander, published subsequently to 

 that of Eichwald but unknown in England, and in which all the transition fossils of the neigh- 

 bourhood of St. Petersburgh are elaborately described, I ascertained that the Cryptonymus 

 Rosenbergii is the Illoenus crassicauda, Dalm., a fossil quite distinct from our specimen 1 . 



I have therefore ventured to consider this trilobite a new genus, and to name it Bumastus, 

 the specific name being derived from the locality where it was found. The peculiarity of 

 structure of its surface has not I believe been noticed by the Swedish or Russian authors as 

 belonging to any trilobite described by them, nor can I detect it in a specimen of Illcenus 

 crassicauda, collected by M. Brongniart near Linkoping in Sweden, and lent to me by Mr. C. 

 Stokes, though a specimen supposed to be from Russia, apparently identical with our Bu- 

 mastus Barriensis, and having precisely the same surface, was pointed out to me in the British 

 Museum by Mr. Konig. Being acquainted with one form only of Bumastus, it is possible 



1 The work of Pander, " Beitrdge zur Geognosie des Russischen Reiches, 1830," was obligingly sent to me by 

 the Baron de Meyendorf, through my friend M. de Verneuil. I shall allude to this work again at the conclu- 

 sion of this chapter. 



