126 E. A. ANDREWS. 



from above, successive optical sections presented the outlines seen 

 in Fig. 4 : the heavy black line is the outline of the apex above 

 the receptacle ; the line No. 2 is the circumference of the annulus 

 around the level (2) of Fig. 3, and in the receptacle is repre- 

 sented by dotted lines ; the line 3 shows the maximum thickness 

 from before back, also the greatest depth of the median groove ; 

 finally the line 4, Figs. 3 and 4, shows the great width of the 

 annulus and its compression from before back, near its base. An 

 anterior view of this same annulus was like Fig. 3 but with the 

 sides reversed and with the annulus and median groove invisible 

 until the annulus was made transparent, when they were seen 

 through its substance. 



By study of annuli cleared whole and also cut into sections the 

 internal structure was seen to agree with what was made out for 

 other annuli (9). Thus a lengthwise section (Fig. 8), shows a 

 thick exoskeleton lined by nucleated epidermis and a central mass 

 of areolar connective tissue full of blood sinuses and vessels with 

 no discovered musculature. The shell on the straighter posterior 

 face seems thicker than on the more protuberant anterior face. 

 The infolding near the upper end was the part of the receptacle 

 of this dextral annulus, cut near the right face. 



As a protruding mass of connective tissue covered with thick 

 exoskeleton the annulus in other crayfishes forms a stiff plate 

 more or less movable since the exoskeleton round about it is less 

 thick and more pliable, but in Cambams montezumce the mobility 

 is apparently enhanced from the fact that the great height of the 

 mass and its narrowed base make it easy to rock it back and 

 forth. The convex front face of the annulus fits against the hol- 

 lowed out sternal plate of the somite bearing the fourth legs, as 

 s imperfectly indicated in Fig. 1 . 



As the spine on the middle of the sternum behind the annulus 

 is on that somite of the thorax which can be independently 

 moved by the animal, it may be that the annulus is sometimes 

 caught between the spine that enters its posterior groove and the 

 solid sterna in front of it and subjected to pressure and this may 

 be a means of emptying the receptacle. 



The receptacle itself proves to be made precisely along the 

 same lines as in the other crayfish in which it has been described 



