On Fresh-water Entomostraca. 19 



Description of the female. 



The largest specimens examined attain a length of 1,30 

 mm., and this form accordingly grows to a somewhat larger 

 size than M. propinqua, which, as a rule, does not exceed 

 a length of 1 mm. 



The general form of the body (see fig. 1) is that cha- 

 racteristic of the genus, the head being very sharply marked 

 off from the shell by a deep dorsal depression. 



The form of the shell appears rather variable, according 

 to the degree of distention of its dorsal part with ova or 

 embryos. Sometimes this part is quite enormously distended, 

 so as to form an almost globular expansion, sharply defined 

 from the valvular part of the shell, and this is gener- 

 ally the case with all the individuals of the earlier gener- 

 ations. The valvular part of the shell, however, preserves 

 its shape unaltered, in all specimens being comparatively 

 small, so as not fully to obtect the tail, a greater part of 

 which is always seen to project freely beyond the shell 

 posteriorly. At the junction between the dorsal and valvular 

 parts, the shell projects posteriorly as a short and obtuse 

 prominence, below which the posterior edges ^appear slightly 

 incurved. The inferior edges of the valves are nearly 

 straight, and join by a perfectly even curve both the anterior 

 and posterior edges. They are clothed with small marginal 

 hairs, which are more distinct in the anterior part, gradu- 

 ally disappearing behind. 



The head is comparatively small and somewhat pro- 

 cumbent without any trace of a dorsal crest, being evenly 

 vaulted above. Just above the ocular region, a slight sinus 

 may be traced, but this sinus is not nearly so pronounced 

 as in M. propinqva. The frontal, or ocular part is but slightly 



