Previous to 1923, knowledge of Cambrian Aglaspida was limited to the published accounts of a few fragments of Aglaspis barrandei (Hall, 1863) and of the dorsal shield of “Aglaspis” eatoni (erroneously presented by Whitfield, (1880erroneously presented by Whitfield, (1882)). Excellent specimens in the possession of the National Museum had not received published notice, and the writer shared the general impression that these organisms were very rare. While in attendance at the University of Wisconsin from 1923 to 1925, however, he was employed to make collections from the Cambrian and Ordovician rocks of southwest Wisconsin, in the course of which increasing numbers of aglaspids were accumulated. These endeavors finally culminated in the discovery, late in 1924, of a locality particularly rich in Merostomata, where a single parting in a relatively limited exposure yielded more than 300 examples including several complete dorsal shields and a large Aglaspis preserving the jointed appendages. (See Figure 6.) In 1926, the Milwaukee Public Museum, coöperating with the National Museum, afforded the writer the opportunity of working on the aglaspid material at the latter institution, under the tutelage of Dr. C. E. Resser and Dr. E. O. Ulrich. Much of the photographing was done at that time, and a first draft of the descriptions was prepared. From 1929 to 1935, while acting as geological curator at the University of Wisconsin, the writer was afforded further opportunity for making collections and preparing manuscript. The writer’s aglaspid collection and that of the University of Wisconsin were gathered during this period. In . . .