THE ANNULUS OF A MEXICAN CRAYFISH. 



125 



the right as in Fig. 2 and the other forty-four on the left. In 

 these left-handed females the posterior side of the annulus ap- 

 peared as in Fig. 3. Leaving out of account the differences in 

 height and width and the altitude of the receptacle, which are 

 diverse in individuals either dextral or sinistral, the annulus in Fig. 

 3 is the mirror image of that in Fig. 2 : the abrupt and the slop- 



Fig. 4. 



ing sides and the position of the apex are reversed and this is 

 obviously associated with the fact that the receptacle is on op- 

 posite sides in the two cases. 



The real shape of the annulus in either 

 sinistral or dextral forms is seen only 

 after observation from different points of 

 view. The annulus is wide from side to 

 side and compressed from before back, 

 very protuberant in front and less so be- 

 hind and has a very small base, which 

 accounts for the movability noticed by 

 Faxon. A view of the sinistral annulus 

 (Fig. 3) taken from the side that bears 

 the receptacle is shown in Fig. 5, which 

 emphasizes the very narrow base and the 

 greatly protruding anterior face. The 

 apex is also seen to rapidly taper. The 

 receptacle obviously extends around onto 

 this lateral side above the level of the 



central groove, which is indicated in broken lines as seen through 

 the side. Upon looking down upon this same sinistral annulus 



Fig. 5. 



