CRUSTACEA. 



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

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

 tors the seUents 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 m the 

 Cyclopid*, 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., IH. Two pairs of antennae. 



« « IV. Mandibles. 



» " V., VI. Two pairs of maxillae. 



tt it 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- 

 illa\ 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 maxillae. 



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



" " XII. Eleventh pair of branchial legs. 



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



to which genus the Branchipus has relations in general form. 



This snbjeet 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- 



