MUSEUM OF COMPARATIVE ZOOLOGY. G5 



the young, and the true septa have a much smaller comparative area 

 than in the adult. 



The outlines and coarser shading of all the figures were drawn with 

 the aid of the Camera Lucida hy me, and these were worked over, and 

 artistically finished with the greatest care by Mr. Konopicky, Draughts- 

 man to the Museum. These outlines were often repeated and verified 

 in obscure or doubtful cases, and the camera drawing invariably cor- 

 rected by the careful study of the more minute structural details. The 

 statement of the number of diameters to which a specimen has been 

 magnified does not show that this power alone was used ; frequently in 

 working out the minor details much higher powers were employed, the 

 results being noted in the general outlines already drawn with a lower 

 power. 



The objectives employed were made by Tolles & Wales, in their 

 best manner, the glass most relied upon being a one-half inch of re- 

 markably good definition, and working distance made by the first 

 named. 



The enlargement of the figures was measured by a direct compari- 

 son of the Camera Lucida image, at a distance of ten inches from the 

 eye-piece with the size of the object on the stage as shown by a gradu- 

 ated ruler, or in the higher powers by a stage micrometer. 



As others may possibly desire to repeat these observations, I will 

 conclude this preface with a few words upon the preparation of speci- 

 mens intended for examination. 



Those specimens in which the shell is replaced by iron pyrites or in 

 which the matrix is stained with iron, are seldom well suited for the 

 observation of the very earliest period. The centre does not generally, 

 in my experience, break out as well as in those which are fossilized 

 by the ordinary Carbonate of Lime, and they are too opaque. Speci- 

 mens with which I have been most successful are found in the hard 

 dark blue limestones. The centres of these are usually filled with 

 translucent carbonate of lime even when the older parts of the whorls 

 are opaque. 



The first process of reduction may be generally effected by breaking 

 away the whorls with chisels of suitable size-, care being taken not to 

 strike too near the centre; then the young shell should be bedded in 

 Canada balsam which has not been overheated. If scorched, the 

 balsam is apt to give way in large splinters and fly off the slide, carry- 



VOL. III. 5 



