TRILOBITES — WHITTINGTON 407 



series have also been obtained from limestones in Utah and Nevada, 

 where they are preserved also by silicification. From other areas 

 and countries, in shales and siltstones, have come size series of articu- 

 lated exoskeletons, a notable example being those described from 

 Czechoslovakia more than a hundred years ago by J. Barrande. 



It is extremely rare to find parts of a trilobite preserved other than 

 the exoskeleton. This is presumably because the exoskeleton was 

 strengthened by secretion of mineral matter, but the covering of the 

 antennules and other appendages was not so reinforced. From a few 

 localities, the most important being in North America, remains of 

 appendages are known. An early discovery, amiomiced in 1876, was 

 made by Charles D. Walcott (later the Secretary of the Smithsonian 

 Institution) in a limestone bed near Trenton Falls, N.Y. Spurred 

 on by his memory of the enthusiasm of Louis Agassiz, Walcott obtained 

 over 3,500 entire trilobites, in a few of which the appendages were 

 preserved. Walcott cut thin sections of these specimens, and demon- 

 strated clearly that trilobites possessed jointed appendages. 



A few years later, W. S. Valiant, then curator of the museum at 

 Kutgers College, picked up a loose piece of rock near Rome, N.Y., 

 which contained a trilobite with appendages preserved by having been 

 infilled with pyrite. A patient 8-year search resulted in the discovery 

 in 1892 of the dark shale layer, less than 1 centimeter thick, from which 

 Valiant's loose specimen had come. The formation contained hun- 

 dreds of similar specimens. Delicate excavations of these fossils 

 were made by Prof. C. E. Beecher of Yale University, but he died 

 while still working on a drawing of one of his remarkable prepara- 

 tions. His student, Percy E. Eaymond, took up the work and wrote 

 an epic monograph concerned with the nature of trilobite appendages. 



Long before this monograph was completed, Walcott had made 

 another sensational discovery, this time in the Burgess Shale — a 

 formation of Middle Cambrian age — near Field, British Columbia. 

 A great variety of arthropods are preserved in these shales, including 

 trilobites with the appendages actually visible as a thin silvery film 

 extending out beyond the margins of the exoskeleton (pi. 2). 



No finds of comparable richness have been made since these early 

 days, and advances in our knowledge have come from the applica- 

 tion of more refuied tecliniques. An example of such an investigation 

 is that made by Prof. Leif St0rmer of the University of Oslo, who 

 came to the United States in 1931 and worked with fragments of Wal- 

 cott's original material from Trenton Falls. St0rmer ground a series 

 of sections, parallel to each other and a small distance apart, through 

 an enrolled specimen. An enlarged drawing of each section was 

 made, and each drawing was traced on a sheet of wax. The thick- 

 ness of the wax sheets wa.s proportional to the enlargement of the 



