188 Correspondence — Professor A. von Koenen. 



rests conformably on Llandovery rocks below, and passes up without 

 a break into Wenlock beds above. This rock - series is strati- 

 grapbically continuous from base to summit, and includes the four 

 divisions of the Brynmair, Gelli, Talerddig, and Dolgau Groups, 

 which, while they possess distinctive features of their own, are 

 bound together by common palseontological characters. The lowest 

 two, namely, the Brynmair and Gelli Groups, consist mainly of 

 grey shales and mudstones with beds of thin flags, which increase 

 in number and thickness as one ascends the sequence. The 

 Talerddig Group is distinctly an arenaceous one, and contains 

 numerous bands of thick grit which are generally massed together 

 at four or five distinct horizons. The highest member of the 

 series, the Dolgau Group, answering to the local ' Tarannon Shales' 

 of the Geological Survey, consists of pale-grey and pui'ple mudstones, 

 the latter being inconstant in number and thickness in diflferent 

 parts of the district. 



The strata of the overlying Wenlock Series present all the 

 ■characters of the Denbigh Grits and Flags of North Wales. Some 

 2,000 feet are developed in this district, the upper beds consisting 

 of grits and flags, while the lower are mainly shales and mudstones. 



The Llandovery Series, which underlies the Tarannon Series, 

 has, at present, been recognized only in the western part of the 

 district, namely, in the valley of tlie Tw^nnyn, and its rocks are 

 brought to the surface by an anticlinal fold. Representatives 

 of nearly the whole of the Llandovery beds have been met with at 

 different localities, and five distinct graptolitic zones have been 

 recognized. The rocks, which consist almost entirely of soft shales 

 and mudstones, are probably not more than 400 feet thick. 



A comparison of the graptolitic lists shows that the Tarannon 

 Series, as here defined, corresponds almost exactly with the Gala 

 or Queensberry Group of the South of Scotland, includes all the 

 palfeoutological zones hitherto assigned to the Tarannon, and fills 

 up the whole period intervening between the Llandovery below and 

 the Wenlock above. It includes the extreme beds which have been 

 mapped as Tarannon by the Geological Survey in Wales ; and in 

 the Tarannon District, at all events, the thickness of the series is 

 equivalent to its maximum development elsewhere. 



coI^K,ESI=OI^^z^EI^^G:B. 



SMALL FOSSIL SHELLS PEESERVED WITHIN THE INTERIOR 



OF LARGER ONES AND IN THE BODY-CHAMBER OF 



CEPHALOPODS. 



Sir, — It is a well-known fact to all collectors of fossils and 



practical geologists, that in the interior or body-cavity of larger 



fossils very often smaller ones may be found splendidly preserved, 



which otherwise are not to be got at all, or only in a very poor 



state, being crushed or weathered or wholly destroyed. In this way, 



when I worked, especially on the Tertiaries, I obtained the very 



