Walcott considers _Waptia_ as a transitional form between the Branchiopoda and the Malacostraca. =Yohoia tenuis= Walcott. Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1912, p. 172, pl. 29, figs. 7-13. This species, though incompletely known, has several interesting characteristics. The head shows, quite plainly in some specimens, the five segments of which it is composed. The eyes are small, situated in a niche between the first and second segments, and are described as being pedunculate. The eight segments of the thorax all show short triangular pleural extensions, somewhat like those of _Remopleurides_ or _Robergia_. The abdomen consists of four cylindrical segments, the last with a pair of expanded caudal rami. The antennules appear to be short, while the antennas are large, with several segments, ending in three spines, and apparently adapted for serving as claspers in the male. The third, fourth, and fifth pairs of cephalic appendages are short, tapering, endopodite-like legs similar to those of _Burgessia_. The appendages of the thorax are not well preserved, and there seem to be none on the abdomen. This species is referred by Walcott to the Anostraca. =Opabina regalis= Walcott. Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1912, p. 167, pl. 27, fig. 6; pl. 28, fig. 1. This most remarkably specialized anostracan is not well enough known to allow comparison to be made with other contemporaneous Crustacea, but it is worthy of mention. There is no carapace, the eyes are pedunculated, thorax and abdomen are not differentiated, and the telson is a broad, elongate, spatulate plate. There seem to be sexual differences in the form of the anterior cephalic and caudal appendages, but this is not fully established. The most remarkable feature is the long, large, median cephalic appendage which is so suggestive of the proboscis of the recent _Thamnocephalus platyurus_ Packard. The appendages are not well enough preserved to permit a determination as to whether they are schizopodal or phyllopodan. _Summary._ Walcott referred _Burgessia_ and _Waptia_ to new families under the Notostraca, while _Yohoia_ and _Opabina_ were placed with the Anostraca. Except for the development of the carapace, there is a striking similarity between _Waptia_ and _Yohoia_, serving to connect the two groups.