﻿OGYGIA. 



127 



denly tumid, so as to undulate the flat marginal band. Incurved under portion of the 

 tail (caudal fascia) narrow, closely striate, not indented by the axis ; the upper side has a 

 striate band of the same width, 1 which is indented by every rib of the tail. The fascia is 

 continued (of the same width) along the ends of the pleura;, and the stria) run in the 

 same direction, while on the upper surface they are transverse, as above said, and reach 

 further inwards. Numerous fine stria;, arched upwards, cover the axis. 



" Variations. — This fine Trilobitc varies a little in convexity, but greatly more in pro- 

 portionate length and in the width of the axis ; this variation appears to be chiefly due to 

 sex. If we are right in referring the long form to the $ , we must allow the PI. XIV, 

 fig. 3, and PI. XV, fig. 1, to be males, and the remainder female specimens. The 

 difference is rather extreme in this species. But there is also another variety. PI. XV, 

 fig. 5 (a ? form), has a pair of tubercles on each segment of the axis ; they are distinct 

 and strong in this specimen, and are faintly seen in some others, both ? and <$ forms. 

 It might be called var. tuberculata. 



"0. Buc/iii is known from the French species Asaphus Guettardi by its less elongate 

 and more truly oval shape, and the greater width of the pleura in proportion to the axis. 

 In A. Guettardi they are but half as wide again. The axis of the tail is longer ; has 

 thirteen instead of nine ribs ; and is abrupt, not attenuated, at the tip. In A. Guettardi 

 it extends but three quarters the length. Our fossil has also duplicate, bent, and more 

 numerous side-furrows ; the French fossil has but eight or nine straight simple ones ; the 

 labrum also is subcorneal, not dilated laterally. But the general aspect of the two fossils 

 is much alike. 



"0. dilatata, Briinnich (not of Portlock), which seems to have been frequently esteemed 

 a variety of 0. Buchii, has the glabella short, with its lobes crowded down towards the 

 base and the eyes remote. The facial suture behind the eyes appears much less arched 

 in Sars' figure. He also describes the tail as with ten 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 

 0. dilatata is within the front margin on the upper side, but in 0. Buchii is along the 

 edge itself," as in Asajihus tyrannus, soon to be figured. 



" The earliest mention we can find of Trilobites is concerning this species, and is that 

 which all writers on these fossils have quoted. Dr. Edward Llhwyd, in a letter to Dr. 

 Martin Lister, of the Royal Society (1G98), writes 'concerning several regularly figured 

 stones lately found by him.' ' The fifteenth,' he says, ' we found near the Lhan Deilo, 

 in Caermarthenshire, in great plenty ; it must doubtless be referred to the sceleton of some 

 flat fish ;' but he remarks a few lines after, ' Not that these or any other marine terrestrial 

 bodies were really parts or exuvia; of animals ; but they bear the same relation to them as 

 fossil shells to marine ones,' &c. This latter opinion he maintains in his ' Lithojjhjlacii 



1 In maDy of the Asaphidce the caudal fascia is much wider than the upper striated band. 



