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E. EXPLANATION OF PLATES. 



The figures are all photographs, in a few instances from actual specimens, but, in the case of all the 

 Girvan fossils and most of the others, taken from squeezes of the natural imprints. The photographs were 

 taken on a scale half as large again as that of the completed plate, and, after being retouched where 

 necessary, were reduced when photographed for the collotype. Except for one or two kindly made for me 

 by Mr F. W. Reader, all the photographs were taken by Mr H. G. Herring, to whose skill and pains I 

 am much indebted. The difficulty of making good enlarged photographs from this class of objects is no 

 small one, but after a few trials considerable success was attained by the use of an Aldis anastigmat lens, 

 Ser. II., No. 0, Focus 3 in., with the diaphragm stop reduced to its minimum. Most of the objects were 

 illuminated by two enclosed arc lamps. 



The squeezes were made in a special composition of gutta-percha and wax, which permits the repro- 

 duction of detail so minute as the micro-structure of the stereom, and at the same time can be withdrawn 

 from deeply undercut imprints. 



The retouching was done chiefly with Japanese water-colours, which do not obscure the detail of the 

 photographic print. It was in the main confined to the darkening of the background and the intensification 

 of suture-lines. 



The production of the collotype plates was carried out by the London Stereoscopic Co., Ltd., whose 

 able stafl' deserve my thanks for the care they exercised. 



Thanks are due to the Council of the Royal Society, London, for a grant in aid of the publication of the 

 plates required for this paper. 



