Z PROCEEDINGS OF THE NATIONAL MUSEUM vol. 78 



untouched photograph. Practically all of the material studied 

 occurs in a dark matrix, and it was necessary to whiten the specimens 

 by the ammonium-chloride process before suitable photographs could 

 be secured. This process has also proved almost invaluable in the 

 study of the specimens, for by the application of the thin, smooth, 

 white coating of ammonium chloride characters which are obscure 

 in the unwhitened specimen flash out into relief. 



In the following pages we are compelled to record mistakes due 

 mainly to incomplete preparation of material, all of which might 

 have been avoided had some process for clearing up the specific char- 

 acters been available. In some cases the posterior side was mistaken 

 for the dorsal edge of the valve; frequently the outlines of the valves 

 were incorrectly figured, the true edges being hidden under the 

 matrix, as subsequent investigation showed. Even when perfectly 

 prepared great care must be exercised in the study of these small 

 Crustacea, for their thin, flexible, and membranaceous sheUs are very 

 liable to distortion or crushing. Numerous specimens are sometimes 

 necessary to determine the true form of a species, and in this respect, 

 as noted above, the present wiiters have been particularly fortunate 

 in having at their service the great Cambrian collection accumulated 

 by Doctor Walcott during a half century of unremitting effort. 



As before stated, only the small ostracodelike bivalved Crustacea 

 of the Cambrian are considered in this paper. The great majority 

 of the species are arranged in three closely related families, the 

 Bradoriidae, Beyrichonidae, and Indianidae, while the remaining 

 forms are referred to the recent Limnadiidae or have been left uncer- 

 tain. Our studies upon these Crustacea were well under way in 1913, 

 so that the junior author was then able to definitely place these so- 

 called Cambrian Ostracoda in the Order Conchostraca of the old 

 division Phyllopoda in his discussion of the Branchiopoda in the 

 second edition of the Zittel-Eastman Textbook of Paleontology. 



The larger Cambrian Crustacea classified under Isoxys, Hymen- 

 ocaris, and various genera proposed by Walcott are not considered 

 in the present paper, but their position in the classification is indicated 

 in the table on a succeeding page. 



The Cambrian bivalve Crustacea referred by Matthew and others 

 to the Ostracoda certainly do not belong to that superorder. In all 

 of the forms studied by us, with the exception possibly of certain 

 species placed in the emended genus Indiana, the main muscle spot 

 is located close to the anterocardinal angle just behind and beneath 

 the ocular tubercle, whereas in the Ostracoda what is regarded as the 

 corresponding scar is located somewhere near the middle of the valves. 

 In at least one of the species of Indiana — /. primaeva {Leperditiaf 

 primaeva, Matthew) — an obscure subcircular scar is located very 

 near the center of the valves. In all, however, the composition of 



