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THE AMERICAN NATURALIST [Vol. XLI 



In this departure from ancestral conditions C. affinis has gone 

 farther than Astaciis in the following respects. In the first stage 

 and in the second stage the telson is more reduced and both pairs 

 of antennae are more simple and to some extent this is also true in 

 the other species of Cambarus here described. Thus in tlie first 

 stage, Astacus has 50 to 66 spines along both the posterior and 

 lateral edges of the telson while the three species of Cambarus 

 have spines only upon the posterior edge and they are less than 30. 

 Astacus also has in the first stage five segments in endopodite and 

 exopodite of the first antenna and .")() in the filament of the second 

 antemia while the three species of ( anilfarn. have but four in the 

 first case and 2.) to '.^o in the last. 



In the first larval stage the three species of Cambarus thus 

 agree amongst themselves and depart from Astacus in the direction 

 of simplicity which is presumed to be a secondary reduction in 

 connection with protected life upon the mother. 



In the second larval stage C. affims alone has spines merely 

 and no plumose seta? upon its telson and is thus most remote from 

 fitness for the active life of its ancestors. In the second stage 

 Astacus is most like a free form in having its telson fringed with 

 much more perfect and ninneron. j)luin()se seta; than are found in 

 C. clarkii or (\ <li,>;/nirs. In Asta-us also the first antenna has its 

 ear-pit well overarched by a row of plninose seta* but in C. darkii 

 there are only 3 or 4 plumes, in ('. (Hixji'tics bnt one i)lunie and in 



second antenna has .")4 s(«gnient>, in ('. (liot/cnrs about 40, in C. 

 clarkii 34 and in ('. alJiii/.s :v.). in ('. (ijjinis alone is there a reten- 

 tion of simple >|)ine> Mich a> occur in the first larval stage, so 

 that the scale of the second antiMnia here still bears no plumes. 



Thus in the second .ta.u'e C. is most removed from Astacus 



but C. diogrNrs and f. r/arkli .lepart h's> from the anceMral Asta- 



