R. M. Field — Roentgen Ray in Paleontology. 549 



some time in their investigation of the internal structures of 

 brachiopods where valves, spires, brachial supports, hinge 

 teeth, have all been preserved by the same mineral, usually cal- 

 cite or its allied form, aragonite. Attempts have been made 

 to section brachiopods in which the internal structure has been 

 preserved, and by a number of such sections to discover the 

 form of the brachial support, form and location of the spires, 

 etc. . . * Such a method is not only tedious but also difficult 

 to interpret in the attempt to delineate the true structures. 

 Obviously, if a method could be evolved for depicting the 

 internal structure of a brachiopod without a preliminary prepar- 

 ation of the specimen, such a method would be of distinct value 

 to the paleontologist in his determination of genera. Figs. 6, 

 7, 8, show the first attempt to prove that the X-ray will deter- 

 mine structure, under favorable conditions, when the replace- 

 ment of both valves and spires has been by siliceous material. 

 Fig. 6 represents the photograph of the actual specimen with 

 pedicle valve removed to show the spires and air space. 

 Figs. 7, 8, are reproductions of the actual results obtained in 

 the skiagraph. The spires are seen to be clearly distinguished 

 from the shadowy outline of the brachial valve through which 

 they have been projected by the ray. Obviously then, the ray 

 will distinguish between siliceous spires and siliceous valves 

 under favorable conditions. Will the ray distinguish between 

 spires and valves under less favorable conditions, namely, under 

 the conditions in which fossil brachiopods are usually found ? 

 Figs. 3, 4, 5 show the duplication of a skiagraph in which the 

 spires are found to be faithfully delineated within the shadows 

 of both valves. Owing to the added thickness of the pedicle 

 valve and want of air space between the spires and the valves 

 the graph is less distinct than in 7, 8, but still plainly recorded 

 on the X-ray plate. It is, therefore, possible to discover, by 

 means of the Roentgen apparatus, the internal structure of a 

 fossil brachiopod when the said structure is entirely hidden 

 from sight. The fossils selected for this experiment were 

 composed of a mineral which is penetrated with more difficulty 

 than calcite or aragonite (see table on previous page), thus the 

 above statement holds good for calciferous replacements. 



Conclusions. 



The writer's experiments have led him to believe that in the 

 majority of cases where internal structures are present in fos- 

 sils they may be discovered by means of the X-ray. Slight 

 differences in density are bound to arise, due to the varying 

 conditions of replacement by crystallization, between the sub- 



* "Internal characteristics of some Mississippian Rhynchonelliform shells," 

 Geol. Society of America, vol. xxi, pp. 497-516. Also by the same author, 

 "Genera of Mississippian loop-bearing Brachiopoda," Jour, of Geol., vol. 

 xix, No. 5,1911. 



