V 



294 Belt— New TriloUtesfrom North Wales. 



III. — On some new Trilobites from the Upper Cambrian Eocks 



OF North Wales. 



By Thomas Belt, F.G.S. 



(PLATE XII., Figs. 3-6.) 



IN the autumn of 1864, Mr. Ezekiel Williamson discovered frag- 

 ments of trilobites in some slaty beds belonging to the '' Lower 

 Lingula Flags " of the Geological Survey, on the right bank of the 

 river Mawddach, a little above its junction with the Eden, and four 

 and a half miles directly north fi-om Dolgelly. Portions of an Olenus 

 and of an Agnostus were found, but in too fragmentary a condition 

 to be determined. Last summer Mr. J. Chamberlain Barlow, of 

 Birmingham, found the same fossils in great abundance and good pre- 

 servation on both sides of the Mawddach, opposite to Dolmelynllyn. 

 During the present year I have, from these beds, added another 

 species of Agnostus to the scanty fauna ; and, after considerable 

 trouble, in consequence of the rocks being greatly faulted, have 

 been able to determine the true position of this new fossiliferous 

 zone. 



The fossils that have been found are Olenus gihhosus, Wahl., now 

 for the first time recorded as British ; Agnostus nodasus, sp. nov., 

 and a strongly marked variety of Agnostus pisiformis, Lin. The 

 beds containing these fossils lie about midway between the " Mene- 

 vian group " of Mr. Salter, which forms the base of the Upper 

 Cambrian formation, and a thick series of blue slaty beds cha- 

 racterized hj a great abundance of Olenus eataractes, Sal., and 

 Agnostus pisiformis, Lin., and separated from each, above and below, 

 by yellow and yelloAvish grey flaky and flaggy beds that have 

 received from Mr. Salter the local name of " Cwmhesian flags." 



The following diagram will exhibit more clearly the position of 

 these beds. The whole of the strata there shown form less than one- 

 third of the great mass of rocks that have been called "Lingula 

 Flags " by the Geological Survey, and the topmost of them lies 

 several hundi-ed feet below the beds containing the well-known 

 Lingula Davisii. M'Coy. 



o 



60 



Dark blue and blue grey slaty jointed beds about 1200 feet 



thick. 

 Olenus eataractes, Salter, common. Agnostus pisiformis, Lin., 



common. Lingula sp., rare. 



Grey and yellow grey ilaky and flaggy beds — about 600 feet. 

 Agnostus pisiformis, Lin., in upper beds. 



Blue and blue grey jointed beds, about 300 feet. 

 0. gibbosus, Wahl. A, nodosus, sp. nov. A. pisiformis, var. 

 obesus. 



Yellow grey flaky and flaggy beds, with bands of hard grey 

 grit — about 400 feet. 



Dark blue beds — about 500 feet. 



Faradoxides, Conocoryphe, Agnostus, Microdiscus, etc., etc. 

 Menevian group, Salter. 



