664 



TRILOBITES OF THE LOWER SILURIAN ROCKS. 



horde; un sillon pen profond, longitudinal, part ant de son extremity anterieure. Point d'autres 

 tubercules que les oculiformes. Protuberances oculiformes, pen saillantes, non reticulees, 

 angles posterieurs du bouclier, proUmg^ en pointes. Lobes longitudinaux pen saillans. Huit 

 articulations k l'abdomen. 

 Ogygia Murchisonice (n. s.), PL 25. f . 3 a and b. 



In establishing the genus Ogygia, M. Brongniart remarks that although it has a very differ- 

 ent aspect from that of most trilobites, it is not always easily separable from other genera by 

 neatly defined distinctions. 



The best marked character, perhaps, is that of the elongated oval form with nearly balanced 

 extremities, and the prolongation of the buckler on each side, into a slender spike quite sepa- 

 rated from the body. This last-mentioned distinction of the French author is, however, not 

 peculiar to the Ogygia, being, if possible, more strongly marked in the Trinudeus. The deep 

 longitudinal furrow on the corslet of the buckler, may, however, be considered a generic 

 distinction, and is quite apparent in our specimens. In the general outline and in the shape 

 of the buckler, the Silurian species resembles the O. Guettardi, (Brongn.), but differs from it 

 in several respects, particularly in having no appearance of costal divisions in the post ab- 

 domen. 



Loc. The specimens figured were found by Mrs. Murchison, in a black schistose rock at Mount 

 Pleasant, near Caermarthen* These beds lie very low in the Silurian System. Are they of 

 the same age as the black slate of Angers in France, where trilobites of this form have 

 been long known ? 

 Agnostus, Brongniart. Battus, Dalman. 



Gen. Char. — " Corps ellipsoide, hemicylindrique. Bouclier et flancs hordes, a bords un peu 

 r sieve's. Lobe moyen ne pre'sentant que deux divisions transversales d'une seule piece chacune. 

 Deux tubercules glanduleux a la par tie anterieure du corps. " Brongniart. 

 Agnostus piriformis ? Brongn., PI. 25. f. 4 a and b. 



Unable to throw any light on the history of these curious bodies, which some naturalists 

 conceived to be crustaceans in an incipient condition 1 ; I merely figure this species on the 

 same plate as the Asaphus Buchii, to show that its geological position in the British Isles 

 is low in the Silurian System. In Norway, the Agnosti apparently occur in millions, but 

 in our rocks they are much less frequent. 

 Loc. Near Builth. 



Fig. 7- Impressions of crinoidal plates of stems ? in the Llandeilo flags. If these are doubt- 

 ful, I may add that perfect crinoidal plates occur in the Llandeilo limestone at Clog-y-frain, 

 near St. Clears. See PI. 18. f. 5. 



Postscript. — I received the work of Pander, alluded to p. 658., at too late a period to enable me 

 to profit much by his views concerning the original structure of the Trilobite, or the adaptations 

 of the tegumentary skeleton of the animal to its habits, into the consideration of which he enters 



1 The notion of the Agnostus being a young trilobite, can have no real foundation; for, as Dalman remarks, 

 we see the most perfect forms of certain species of trilobites, and usually of great size, not larger than peas. 

 Eichwald has conjectured the Agnosti to be eggs of Orthoceras. According to Kloden they are related to the 

 genus Limulus. 



