206 STUDIES IN EVOLUTION 



homologous positions with other Crustacea only when there 

 is some evident reason for so doing. 



First Pair of Biramous Appendages, or Posterior Antennae. — 

 The second pair of appendages, corresponding to the posterior 

 antennae, are attached to the head at each side of the glabella, 

 on a line with the extremity of the hypostoma. They are 

 apparentl}^ biramous, and thus agree with the second pair of 

 nauplian limbs and with the typical posterior antennte of many 

 Entomostraca and Malacostraca. They may be compared with 

 the posterior antenna? in Euphausia jjellucida, one of the schiz- 

 opods, especially with the Furcilia and Oyrtopia stages. The 

 details of the endopodite and exopodite are not clearly shown. 

 The former is more commonly preserved, and its distal joint 

 extends just beyond the edge of the carapace. The coxopo- 

 dite is developed into a triangular plate, the inner angle 

 carrying a masticatory ridge, the whole extending about 

 three-fourths the distance from the side of the glabella to 

 the median line, just below the hypostoma, and directed 

 obliquely backward (Plate VIIT, figures 8-11). 



Second Pair of Biramous Appendages, or Mandibles. — The 

 appendages here correlated with the mandibles are immedi- 

 ately behind the first pair of biramous limbs. The proximal por- 

 tion, or coxopodite, is similar in form to the preceding, though 

 somewhat smaller, and overlapping its basal part. The palps, 

 or endopodial and exopodial branches, have not been distinctly 

 traced, though their presence is indicated on Plate VII, fig- 

 ure 1, where, on the left side, there are endopodites and exop- 

 odites in sufficient number for each appendage of the head. 

 That these should be referred to the cephalic limbs is further 

 indicated by their being in advance of the endopodite, which 

 manifestly pertains to the first thoracic segment. The inner 

 edge of the mandibles as well as that of the other gnathobases 

 of the head is apparently finely denticulate, as shown on Plate 

 VII, figure 1, and Plate VIII, figure 2. 



Third and Fourth Biramous Appendages, or Maxillae. — 

 Following the appendages referred to the mandibles are two 

 pairs of strong limbs, with broad plate-like basal portions, or 



