14 Wyatt-Edgell — On the Oenera of Trilobites. 



and I tliink ttey are more likely to be the equivalents of this group, 

 than of any other of the Millstone Grits. 



The presence of Mountain Limestone Fossils, and the band of 

 earthy limestone in No. 9, is in favour of the Yoredale affinities of 

 these beds. Such remains and like limestones are common in the 

 lowest, and perhaps occur sparingly in the upper, Yoredale Groups 

 of Lancashire, Staffordshire, Derbyshire, and the West Eiding of 

 Yorkshire ; but I do not know a single case of any such beds in the 

 Millstone Grit of those districts. 



I am inclined to think then, that the Millstone Grit is represented 

 in North Wales by its uppermost member, the Eough Eock, alone : 

 and that all the lower beds have thinned away before we reach 

 Flintshire. This agrees with what we know happens elsewhere to 

 this sub-formation as it is traced southwards. 



It also seems that the sandstones underlying the Eough Eock 

 are, in mineral character and fossils, more closely related to the 

 Yoredale than to the Millstone group ; and that in many respects 

 they agree very well with the middle division of the Yoredale beds 

 as seen in North Staffordshire. 



Valle Crucis Abbey and the bridge at Llangollen are built, I think, 

 of some of these, supposed Yoredale, sandstones : they have stood 

 weather admirably, the carving and tool marks being as fresh as 

 when first cut. 



lY. — On the Genera of Tkilobites Asaprus and Ogtgia and 

 THE Sub-genus Fttchoptge. 



By the late 11. Wyatt-Edgell,' Esq., 13tli Light Infantry. 



BY the able investigations of Mr. J. W. Salter, the various species 

 of British TrUobites are now being systematically described, and 

 referred to the genera to which they properly belong,^ the AsapMda 

 with the rest ; but there seems to be still some doubt as to the true 

 differences between AsapJms and Ogygia. The feature apparently con- 

 sidered distinctive in the masterly work alluded to, is the form of 

 the labrum ; which, if furcate, is to be set down to Asaphus ; if ob- 

 tusely pointed, to be the distinguishing mark of Ogygia. How 

 Ogygia peltata (Woodcut, Fig. 5) can be reconciled to this division, 

 I do not see ; but the main difficulty in the way of this classification 

 is the sub-genus Ptychopyge of Angelin — distinguished by "having 

 the facial suture within the margin in front." This sub -genus 

 Mr. Salter shows to be represented in Britain by Ogygia Corndensis, 

 Murchison (Woodcut, Fig. 1) : it is called Asaphus by Angelin, 

 and has the wide axis, short and broad pleural furrows and simple 

 caudal ribs of that genus. In addition to these features, the 

 course of the facial suture finds a parallel in the sub-genera of 

 Asaphus, Isotelus, (Dekay) and Cryptonymus (Eichwald) ; Mr. Salter 



1 From a MS. found amongst the author's papers. 



2 See Mr. Salter's monographs on British Trilobites, puhlished hy the Paloeonto- 

 graphical Society. Part I., 1864; Part II., 1865; Part III., 1866. It is to these 

 two latter parts that this paper specially refers. — Edit. 



