EVOLUTION OF FOSSIL CEPHALOPODA. 251 



has shown us the way; Branco, by his studies of the 

 larval stages of ammonoids, has accumulated a great 

 mass of accurate data that can be used with confidence, 

 even by the student that rejects his theories as to classi- 

 fication. And Karpinsky, by using the methods and 

 principles discovered by these naturalists, has worked 

 out the genealogy of one of the chief stocks of the 

 earlier ammonites. This way lies the truth, and not in 

 groundless speculations such as many students of cepha- 

 lopods are prone to indulge in. 



In order to succeed, one must select material with 



great care, preferably limestone that is soft but not so 



weathered as to crumble; nor so brittle 



f as t0 shatter - One>s fin S er nail and 



workh 



some steel dental chisels are all the 



tools needed for breaking off the outer whorls of young 

 ammonites. A microscope with thirty diameters magni- 

 fying power is the most satisfactory, although higher 

 powers are occasionally needed. For studying surface 

 markings a strong pocket lens is usually sufficient; the 

 specimen should then be placed dry on white cardboard. 

 For observing the sutures, or shape of the whorls, the 

 specimen should be placed on cardboard in a drop of 

 water, spread out so as not to distort the object. The 

 water, being slightly viscous, will also hold the small 

 object in any position. For taking measurements a 

 micrometer eyepiece is needed, especially in drawing, 

 for the camera lucida is not very satisfactory for draw- 

 ing opaque objects. Sections can easily be cut by 

 grinding with emery powder on a glass plate. 



The accompanying illustrations will give an idea of 

 how the facts are ascertained. A number of well-pre- 

 served adults of a species are selected, and the outer 

 coils are pulled off piece at a time under water, until a 

 complete series is obtained, representing every change 



