BRITISH FOSSILS. 3 



Affinities. From O. Guettardi it is known by its less elongate and 

 more oval shape, and greater width of pleurae in proportion to the axis ; in 

 O. Guettardi they are but once and a-half the width. The axis of the 

 tail is longer, and has 13 instead of 9 ribs, and is abrupt, not attenuated 

 at the tip ; in O. Guettardi it extends but three-quarters the length. 

 It has, also, duplicate, bent, and more numerous side furrows ; the other 

 has but eight or nine straight simple ones ; the hypostome also is sub- 

 conical, not dilated laterally. O. dilatata, Brunnich (not of Portlock), 

 which seems to have been frequently held as a variety of O. Buchii, has 

 the glabella short, with its lobes crowded down towards the lower part, 

 and the eyes remote. The facial suture behind the eyes appears much 

 less arched in Sars' figure. He also describes the tail as with 10 ribs, 

 separated by broad furrows. The labrum of 0. dilatata, according to 

 Sars' figure, is but slightly different from that of the British species ; 

 but an important difference resides in the facial suture, which, in O. 

 dilatata, is within the front margin on the upper side, but in O. Buchii 

 is along the edge itself, as in Asaphus Tyrannus. 



History. The earliest mention we can find of Trilobites is concern- 

 ing this species, and is that which all writers on these fossils have quoted. 

 Mr. Edward Llhwyd, in a letter to Dr. Martin Lister, of the Royal 

 Society (1698) writes " concerning several regularly figured stones lately 

 found by him." " The 15th," he says, " we found near the Lhan 

 Deilo*, in Caermardhinshire, in great plenty, it must, doubtless, be re- 

 ferred to the sceleton of some flat fish ;" but, a few lines after, " Not 

 that these, or any other marine terrestrial bodies were really parts of 

 exuviae of animals, but they bear the same relation to them as fossil 

 shells to marine ones," &c. This latter opinion he takes care to main- 

 tain in his " Lithophylacii Brittanici Ichnographia " (1699), where he 

 again says, the specimen represents only the skeleton of a sole fish, and 

 wants the tail, and he marvels that the " Piscis Icon " should be raised 

 above the surface of the stone, " ac si verus piscis esset." It is curious 

 that Brongniart should have placed this species in his heterogenous 

 group Asaphus, at the very time he was founding Ogygia, as he ap- 

 pears to have recognized the latter genus more by its marked habit 

 than by any positive characters. He probably meant to unite A. dilatatus 

 with A. Buchii. Dalman distinguished them, but with doubt, in 1826, 

 and gave a figure of the Norwegian fossil from a plaster cast ; and had 

 not Sars, in 1835, given a complete description, and a good figure of 



* Fig. 9, of the same communication, is often quoted as belonging to Trinucleus. We 

 have no doubt it is T. Caractaci, as that is the common species there ( T. fimbriatus is 

 found at Builth) besides, the distinct dots in the fringe identify it with the former species. 

 Fig. 8 appears never to have been quoted, yet it is a tolerable representation of Cybele 

 verrucosa, Dalman, and it has been figured again without a name by Brongniart, plate 4, 

 f. 11, from the same locality, where it abounds, and is the only species of the genus there. 



