550 R. M. Field — Roentgen Ray in Paleontology. 



stances which form the interior and exterior portions of the 

 petrifaction. Slight as these density differences may be, we 

 have seen that they will be registered in the skiagraph. Un- 

 doubtedly, such conditions as the presence of air space between 

 structures and great difference in density between the internal 

 and external structural substances afford the most favorable 

 opportunity for obtaining good results. However, when we 

 realize that the physician is now able to distinguish not only 

 between bone and muscle tissues but also between the inner 

 and outer layer of the epidermis, it would seem possible for 

 the paleontologist to develop the X-ray method to a high stage 

 of usefulness. 



The best results are obtained by studying the original skia- 

 graph as shown in the exposed and developed X-ray plate, for 

 even a print loses greatly in detail. When an enlarged photo- 

 graph of the original skiagraph is made, a print taken from 

 this and the result used for illustrative purposes, it can be 

 readily seen that a great deal of the original detail will be lost. 



In getting " results " in the skiagraphy of inorganic just as 

 of organic substances, a great deal depends on the ability of 

 the investigator to interpret what he sees. Owing to the pres- 

 ent rapid increase in number of Roentgen apparati the expense 

 of making skiagraphs of fossils should be relatively nominal. 

 When we consider the number of small fossils which can be 

 crowded on a 10 X 10 photographic plate and skiagraphed in a 

 few seconds, we realize the value of this method of investiga- 

 tion to the biologist and paleontologist. 



Department of Geology, Harvard University. 



EXPLANATION OF PLATE VIII. 



1. Print from original skiagraph of Cleiothyridina devonica, showing lack 

 of preservation of internal structure. 



2. Eeproduction of skiagraph of Spirifer oweni, showing internal struc- 

 ture. 



3. Photograph (enlarged) of Cleiothyridina sublamellosa with both valves 

 in place. 



4. Reproduction of skiagraph of same specimen. 



5. Print from original skiagraph. 



6. Photograph (enlarged) of Cleiothyridina sublamellosa with spires ex- 

 posed. 



7. Eeproduction of skiagraph of same specimen. 



8. Print from original skiagraph. 



9. Photograph of Triarthrus becki in shale. 



10. Reproduction of skiagraph of same specimen. 



11. Print from original skiagraph. 



(Note.) In the original skiagraphs the denser portions of the fossils are 

 shown by the denser shadows. When a print is made directly from the skia- 

 graph, as shown in figs. 5 and 8, the lights and shadows are naturally 

 reversed. The dark central circles shown in figs. 4 and 7 have nothing to do 

 with the internal structure of the fossil but are the result of the diffusion of 

 the rays, causing a supplementary illumination on the X-ray plate. The two 

 fine, hair-like, black lines in fig. 7 are due to scratches ©n the original 

 skiagraph. 



