BRANCHIOPODA. IO9 
the preservation of the other appendages is unsatisfactory. As illustrated (Walcott, 1912 
A, pi. 27, fig. 5), both endopodites and exopodites appear to be present, and the shaft of 
the exopodite seems to be segmented as in Triarthrus. 
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, pi. 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 triangu- 
lar pleural extensions, somewhat like those of Remopleiirides 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 antennje 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, pi. 27, fig. 6; pi. 28, fig. I. 
This most remarkably specialized anostracan is not well enough known to allow com- 
parison 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 differ- 
entiated, and the telson is a broad, elongate, spatulate plate. There seem to be sexual dif- 
ferences 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 platyitrus 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. 
The Branchiopoda were very highly specialized as early as Middle Cambrian time, the 
carapace of the Notostraca being fully developed and the abdomen limbless. Some (Bur- 
gessia) had numerous segments, but most had relatively few. The most striking point 
about them, however, is that so far as is known none of them had phyllopodan limbs. 
While the preservation is in most cases unsatisfactory, such limbs as are preserved are trilo- 
bite-like, and in the case of Burgessia there can be no possible doubt of the structure. x\n- 
other interesting feature is the retention by Yohoia of vestiges of pleural lobes. The Middle 
