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gneiss upon which these rest, wc are forced to recognise 

 the most ancient of all sedimentary deposits, the venerable 

 Laurentians. From this digression, intended by the lec- 

 turer to recall in a more vivid form to the minds of his 

 audience the chief points treated of on the former oc- 

 casion, he passed on to the business more imme- 

 diately in hand, the members of the Silurian 

 System which overlie the Llandeilo flags and cara- 

 doc standstone. There is now recognised an intermediate 

 formation between the Lower and Upper Silurians, being 

 alike unconformable to both, but being intermediate, as con- 

 taining certain fossils common to both. This is called the 

 Llandovery Series, consisting of an upper and a lower, so 

 called from the little town in Carmarthenshire, where it is 

 most developed. Of this the characteristic fossil is the 

 Pentamerus, a Brachiopod Mollusk, very similar in form to 

 the Terebratula of the chalk and oolite. The upper 

 Silurian system is divided into two well-defined groups 

 called respectively (in ascending order) the Wenlock and 

 the Ludlow formations. These are severally subdivided— 

 the Wenlock, into Denbigh grits and slates, Woolhope lime- 

 stone, Wenlock shale, and Wenlock limestone ; the Ludlow into 

 lower Ludlow, Aymestry limestone, and upper Ludlow. 

 The lecturer then proceeded to exhibit and describe some of 

 the more characteristic fossils, in particular the Halysites 

 and Favosites, the Chain, and Honey-comb corals, of the 

 Wenlock limestone ; the Homalonotus, a Trilobite, charac- 

 teristic of the upper and middle Silurians, and the various 

 genera of Cephalopoda -the Lituitcs, the Phragmoceras, 

 and the Orthoceras-structurally alike, differing only or 

 chiefly in their amount of curvature or straightness. After 

 entering with some minuteness into the conchological 

 structure of the Orthoceras, and its relation to the 

 economy of the animal, he proceeded to describe 

 the " bone-bed," a mass of the debris of fish— teeth, spines, 

 scales, bones, and incerta fragmenta, which forms the 

 uppermost zone of Siluria, and immediately underlies the 



