42 CRUSTACEA. 



thorax is not, a condition improbable. Moreover, as the animal is much 

 like a Schizopod, it has some bearing on this question, that the carapax 

 covers the segments to which the four pairs of natatory legs belong, as 

 if these were thoracic members. Again, the following part of the body, 

 consisting of four segments, resembles an abdomen, and seems to be 

 complete in itself, and has the number of articulations usual in the 

 Cyclopidae, another group to which Nebalia is related ; and the appen- 

 dages below to the first two of these joints are rudimentary and very 

 different from those of the joints preceding. The natatory legs are 

 very closely similar to the four pairs of the Caligidas and Cyclops, ap- 

 pended to the posterior cephalothoracic segments. On this ground 

 we conclude that the eight pairs of branchial plates, and eight seg- 

 ments in Nebalia, belong to the four normal rings, and suggest that 

 the arrangement of the members may be as follows : 



Normal Segment I. Eyes. 



" " II., III. Two pairs of antennas. 



" " IV. Mandibles. 



" " V., VI. Two pairs of maxillss. 



" VII. X. Four segments and eight pairs of branchial legs or plates. 



" XI.-XIV. Four segments and four pairs of natatory legs. 



In Branchipus, the eyes, antenna), mandibles and two pairs of max- 

 illas, belong as usual to six segments. Then there are eleven segments 

 remaining, instead of eight. We have no evidence sufficient for laying- 

 down decisively the true arrangement ; we only suggest the follow- 

 ing: 



Normal Segment I. Eyes. 



" " II., III. Antenna;. 



" " IV. Mandibles. 



" " V., VI. Two pairs of maxillfB. 



VII.-XI. (Five segments) ten pairs of branchial legs. 

 " XII. Eleventh pair of branchial legs. 



" " XIII. and XIV. Obsolete, as in Pontia among the Cyclopidas, 



to which genus the Brauchipus has relations in general form. 



This subject has a high interest, on account of the fact that the 

 earliest Crustacea (Trilobites) were abnormal in number of segments, 

 like the Phyllopoda. They correspond to some modification of the 

 law which now prevails in this class of animals. The basis for con- 



