On Fresh-water Entomostraca. 41 



that genus by the immobility of the head, as in the other 

 species of Mona. 



Description of the female. 



Fully adult, ovigerous specimens do not exceed a length 

 of 0,37 mm., and this form is accordingly of very small size. 



The shell is unusually short and stout, exhibiting, in a 

 lateral view of the animal (fig. 5), a somewhat irregular 

 quadrangular form, with the greatest breadth somewhat in 

 front of the middle. The posterior extremity appears trans- 

 versely truncated, with the upper corner well marked, the 

 lower rounded off. The dorsal margin is boldly arched, 

 forming an abrupt curve at the junction with the head, 

 whereas the ventral one appears straigther, though exhi- 

 biting a slight convexity in front of the middle. The head 

 is semi-erect, and has the dorsal face somewhat flattened, its 

 upper edge being almost straight and declining towards the 

 rostrum ; the latter is rather prominent and, when seen later- 

 ally, acuminate. 



The surface of the shell is very distinctly sculptured, 

 exhibiting in its posterior part a number of obliquely 

 longitudinal lines, which appear somewhat nexuous, thus 

 forming an indistinct reticulation. These lines, in the 

 anterior part of the valves, are crossed by a set of curved, 

 transverse striæ, running parallel to the anterior edges, and 

 rather sharply marked. The inferior edges of the valves 

 are, as usual, densely ciliated. 



The eye is of moderate size, and in its usual situation. 



The ocellus occurs somewhat nearer to it than to the 

 tip of the rostrum, and is of very inferior size. 



The antennulæ exhibit the usual structure, and do not 

 extend to the tip of the rostrum. 



