380 INFERIOR OOLITE AMMONITES. 



mistakes and alterations would have been avoided ; but unfortunately it was not 

 done. It can easily be understood why : a natural reluctance to injure good 

 specimens by breaking away their test ; an equally natural disinclination to 



adze-edged hammer, acid being subsequently applied to remove roughness. If the core be soft and 

 inclined to break away, the test can only be removed by filing or rasping, with subsequent use of acid. 

 If the core be crystalline, acid should not be used ; it tends, for some reason, to obliterate the septal 

 margin. 



When the septa can be seen the outline of the margin may be traced over with a pen holding 

 Indian-ink — in the case o£ black fossils, white paint. For this work a lens will often be required. 

 When the septa are very indistinct, wetting will show them more plainly, and they may then be followed 

 by a coloured pencil. When marked the outline must be traced by means of transparent tracing-paper 

 and a soft pencil : the quality of the tracing-paper is very important. The tracing is then placed 

 over a sheet of tissue-paper which has been rubbed over with red chalk on its lower side. This is laid 

 on a sheet of paper. With a blunt needle the tracing is followed. This gives an outline in red on the 

 paper. This outline must be followed with a pencil, at the same time that such corrections are made 

 as a careful comparison with every curve of the original on the fossil may indicate ; because inequalities 

 of surface cause the pencil to slip in the first tracing, and the marking off afterwards is often not 

 exact. The result, however, should not be much amiss. The errors in my experience arise most 

 frequently from incorrect marking in the first place, owing to the indistinctness of the septal edge : in 

 fossils from calcareous strata this is a serious difficulty. In the subsequent work the errors are slight. 



The above is the method I have used for some years, but lately I have introduced modifications 

 from a hint by M. Nickles.* The tracing is placed in a photographic printing frame, and on it is laid 

 a sheet of ferro-prussiate paper. Exposed to the sun and then washed, this gives the curves of the 

 septal margin in white outline. The white outline is followed by a pen holding Indian-ink, and 

 carefully compared with the original for each curve and denticulation. When the ink is dry the 

 blue colour of the paper may be removed by touching lightly with a brush full of sodium carbonate. 



A further method I have also adopted from a hint given me by Mr. Charles Upton. The 

 alternate chambers are painted red and white : it is a most tedious process, but is often useful when 

 the septa are indistinct. When painted they can be photographed. One error — unless several plates 

 of the same septal margin be taken from different positions — will be a certain foreshortening of the 

 outer and inner parts, particularly of the outer part. Another, and in certain cases more serious error is 

 partly optical and partly photographic. The eye does not allow the two colours to come into actual 

 contact in the work, for if they do the white will be stained red. The consequence is that the part 

 uncovered by white takes photographically, from our yellow or brown fossils, as if it were red — the 

 stained white would do the same. The result is a certain subtraction from the white painted chamber 

 and an addition to the red painted one. In consecutive septa with thin-stemmed lobes there arises a 

 noticeable difference in the width of the stem. 



In connection with the painting process it may be mentioned that it is an admirable method for 

 displaying the septation on the specimens, and it is one which may be commended for public collections, 

 as the different septa in various specimens can be compared at a glance. Further, it is very easy to 

 trace off the septal outline from fossils so painted. 



In regard to the photographic treatment of septa the reader may be referred to the pamphlet by 

 Dr. Nickles {op. cit.). Elaborate directions are given, and to ensure accuracy it is evident that the 



* " Application de la Photographie au dessin des cloisons des Ammonites," ' Bull, de 1' Assoc, des 

 Eleves de l'Ecole des Mines,' 1893. 



