BRITISH FOSSILS. 



Decade XI. Plate I. Figs. 1—5. 



AGNOSTUS PRINCEPS, 



[Genus AGNOSTUS. Beongniart. (Sub-kingdom Articulata. Class Crustacea. 

 Order Trilobita. Family AgnostidjE.) Minute trilobites, with caudal and cephalic 

 shields nearly equal. No eyes, no facial suture. Two body rings. — Ranges from 

 Lingula flags to Caradoc rocks.] 



Diagnosis. A. latus, 7 lineas longus, scutis rotundato-quadratis, 

 ad limbum rugoso-radiatis. Glabella subconica, tuberculo centrali antico, 

 sulcis duobus transversis, lobis basalibus magnis instructa ; sulcoque 

 verticali ad marginem ducto. Annuli corporis valde 7iodosi, Cauda 

 axe magno rotundato, fere per totum caudce extenso ; margine edentulo. 



Synonyms. A. pisiformis, Salter (1859), in Siluria, pp. 45, 53, foss. 4 

 and 9 ; A. princeps, id., * Memoirs Geol. Survey, vol. iii. (ined.), pi. 5. 



Certainly the lowest and most rudimentar}^ form of Trilobite, 

 and greatly resembling in some respects the young stages of higher 

 groups. But Agnostus shows at once its mature character in the 

 possession of a large caudal shield, well developed, and generally 

 quite as large as the head. The surface is sometimes much orna- 

 mented, especially the border, and the lobes are often well marked 

 out both in the cephalic and caudal portions. The leading character 

 of the Trilobite family, the facial suture, is altogether absent, and 

 there are no eyes in any of the species. 



When Brongniart described this fossil for the first time, he 

 evidently could not tell what to make of it. To describe the head 

 and tail as distinct animals was natural enough, but when he 

 turned the hinder part of the head forwards, and suggested that 

 the basal lobes of the glabella might be eyes, and the forehead lobe 

 the abdomen, &c., one feels that he was justified in saying, " On ne 

 salt a quelle classe des regnes organiques le rapporter/' Beyrich 

 first gave the entire form in his treatise, TJeber einige Bohmische 

 Trilobiten," 1845. 



* The reader must understand that the numerous references to Memoirs Geol. Survey, 

 vol. iii. (ined.) throughout this Decade apply to the forthcoming Memoir on the 

 " Geology of Korth Wales," by Professor Ramsay ; with an Appendix on the Fossils by 

 Mr. J. W. Salter. 



[XI. i.] 11 A 



