CRUSTACEA. 
21 
kinate, separated from the cheek by a strong, smooth sulcus; corneal lenses 
abundant, scleral cavities generally hexagonal; the number of lenses vai-ying 
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 fifty for each eye. The variation in the number of lenses, as lar as 
observed, is between thirty and eighty-eight for each eye; the former in a very 
young individual, 12 mm. in length; the latter in an adult of average size, 
measuring 34 mm. in length, and bearing eleven rows of lenses. Eyes show¬ 
ing more than sixty-five 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. 
Pleurae 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 pleurae 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. 
PyoiDiUM 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 pleurce; the latter being broadly 
