CRUSTACEA. 2] 



lunate, separated from the cheek by a strong, smooth sulcus; corneal lenses 

 abundant, scleral cavities generallj hexagonal ; the number of lenses varying 

 greatly with age. Counting from the lower posterior margin diagonally, the 

 number of rows of lenses in normal, mature individuals is ten, in senile or in 

 young individuals often eight or nine. The average number of lenses in nor- 

 mal adults, ascertained by enumeration upon several hundred eyes, is between 

 forty and fifty for each eye. The variation in the number of lenses, as far as 

 observed, is between thirty and eighty-eight for each eye ; the former in a verj 

 young individual, 12 mm. in length; the latter in an adult of average size, 

 measuring 34 nun. in length, and hearing eleven rows of lenses. Eyes show- 

 ing more than sixty-live and less than forty lenses are unusual. 



Thorax sub-quadrate, lateral margins slowly tapering. Surface strongly trilo- 

 bate. 



Axis flattened at the margins, evenly convex in the middle, slightly nar- 

 rower than either lateral lobe, widest at the third or fourth segment, tapering 

 very slowly to the ninth, and thence much more rapidly to the pygidium. 



Pleura flat for about one-third their width from the axis and thence 

 abruptly deflected to the margin. Each segment along the axis shows a 

 slight retral bend from the margins to a point where, upon the under surface, 

 the basal joints of the branchiae, natatory appendages, or visceral supports 

 were attached. Each segment of the pleura 1 bears a furrow which becomes 

 obsolete at the fulcrum ; beyond this point the segment becomes attenuate, 

 presenting scarcely more than a beveled surface of articulation. 



PrciDlUM relatively small; posterior margin the arc of a circle; anterior mar- 

 gin transverse in the middle, angulated toward the sides by the articulating 

 planes. 



Axis rapidly and evenly tapering from the last segment of the thorax, 

 reaching an acute termination just within the posterior margin. 



Pleura broad, depressed-convex, sloping evenly to the posterior margin. 

 In well-preserved examples, nine simple, transverse annulations can be 

 counted upon the axis, and seven upon the pleura'; the latter being broadly 



