88 SUPPLEMENT TO THE 



amhigua as occurring in decomposed limestone near Bakewell ; but I have never seen nor 

 heard, except in the case of the Cambridge specimens, of any examples of Brachiopoda 

 being found amongst our British rocks in which the shell and spirals were silicified whilst 

 the matrix surrounding the spirals was calcareous. Such examples occur not unfre- 

 quently in the Devonian rocks of the United States, and Mr. Whitfield, of the American 

 Museum of Natural History, New York, has developed from these the spirals and their 

 connections in Meristella arcuata, Atri/pa reticularis, &c., with a perfection that leaves 

 nothing to be desired. Indeed, with such specimens a due amount of care is certain to 

 lead to clear and satisfactory results. Mr. Whitfield kindly sent to me a beautiful 

 specimen of Rensellaria, in which the shell and loop were silicified — there being no 

 matrix — and the loop being covered with small crystals of silex. 



" In those cases, comparatively rare, where the shell is found without matrix, and the 

 spirals are coated with crystallised carbonate of lime, there is little chance of revealing 

 the connections, though the spirals themselves may be clearly seen. Mr. Davidson 

 achieved a great success in his original discovery of the spiral arrangements of Athyris 

 under somewhat similar conditions, but the success was without precedent, and 

 seems little likely to have any repetition. The large proportion of the Palaeozoic 

 Brachiopoda are filled with a hard limestone matrix, and whether by sections or by any 

 other method the results in such specimens as to the revealing of the spirals and their 

 connections are very unsatisfactory. 



" It only remains to notice those specimens, previously referred to, in which the shell 

 is partially or wholly filled with a sparry calcareous matrix. It was in such specimens that 

 nearly all the results given in this and in the 'Carboniferous' and ' Devonian Supplements ' 

 were achieved. The specimens having a sparry matrix occur in two general conditions. In 

 some instances, especially in the large Carboniferous examples, the shell is present in a 

 metamorphosed condition of hard limestone. In the majority of cases, especially in Silurian 

 examples, the shell is either absent or changed into the same sparry condition as the matrix. 



" In the former instance the sparry matrix is often revealed by a chip or fracture in 

 the shell. When this is the case the first thing is to remove the shell, and, as the result 

 of my experience, I may say that in most cases the spirals are most easily and effectively 

 revealed by removing the dorsal valve. The shell should not be chipped away, as this 

 would probably fracture the specimen, but it should be placed in a saucer, with the dorsal 

 valve downwards, and then just enough of diluted hydrochloric acid should be put in the 

 saucer to cover the dorsal valve. The acid will probably need renewing several times 

 before the valve is removed so as to show the spar behind it. When the surface of the 

 shell is phcated or irregular it should be first of all filed until it is smooth, otherwise the 

 acid will eat away the surface into pits or hollows. 



" When the shell is absent in the specimens, or changed into the same sparry con- 

 dition as the matrix, of course this preliminary use of the acid is not needed. When 

 nothing but calc-spar intervenes between the surface of the specimens and the spirals then 



