I7O E. A. ANDREWS. 



wings of the fifth legs, and there should be a middle plate pos- 

 terior to the wings of the fourth legs, and as such a plate we 

 may assume the wedge that projects forward from the line 

 joining the fifth wings. 



In the male lobster the wedge of the female is represented by 

 a flat plate that is somewhat grooved along the middle. This 

 plate of the male may very readily be regarded as a middle 

 plate of the fourth leg region shoved back onto the somite of the 

 fifth legs. 



Another mode of judging of the homology of the wedge and 

 its morphological position is a study of the internal skeleton, or 

 endophragmal system. In the crayfish there are transverse 

 ridges on the inside of the shell, between the side wings and the 

 middle plates, and from these arise the long internal bars, or 

 endosternites, that run up into the interior of the body, one on 

 the right and one on the left. In the male it is easily seen that 

 the endosternites come in between the wing plates and the 

 middle plate of the somite bearing the fourth legs and also be- 

 tween the wings and middle plate of the somite of the fifth legs. 

 In the female the same is true except that the wide annulus 

 takes the place of the narrow annular plate of the male. 



The endosternites cannot be taken as exact bounds of the 

 somites, else the middle plate of the last thoracic somite would 

 be reckoned as on the abdomen, which the general conformation 

 renders unlikely. 



In the lobster the extreme narrowness of the sterna brings the 

 endosternites close together at the middle line : but in the male 

 one can see that there is a pair of endosternites between the 

 middle plate and the wings in front of it on the last somite and 

 also between the hollowed out wings of the somite bearing the 

 fourth legs and a middle plate which may be regarded as the 

 annular plate. In the female the only difference is that the 

 peculiar development of the wings and the wedge raises the en- 

 dosternites of the boundary between these plates up onto the top 

 of the seminal receptacle, where they were noticed by Herrick 

 as parts of the endophragmal system. 



It should also be noted that in the male lobster the receptacle 

 is represented by a triangular pit, some two mm. wide, deep 



