STATE GEOLOGIST. Ill 



"there is a set of horizontal strise all over the shell. The post-abdo- 

 ■oien is widest in the middle and almost exactly as in P. denfcieula- 

 tus. The first foot bears a claw such as ordinarily distinguishes 

 the males. 



The only specimens which I have seen were from the Tennessee 

 river, near Waterloo, and near Decatur, in x4.labama. My notes 

 -contain no reference to the minute striations, which eould perhaps- 

 be hardly seen with the instrument employed. The process of the 

 labrum is long and rather acute, the beak moderate, and the pig- 

 ment fleck very large. The markings on the anterior of the valves 

 are irregular and are inter-connected by^cross lines or anastomoses. 

 Ova two. (The genus Anchistropus has a hook upon the first foot, 

 but is like Chydorus.) 



Sp. 9. Pleuroxus aflBliiis. (Sp. n.) 



(Plate H. Fig. 2.) 



A small species with elongated shell and longitudinal striae, form- 

 ing a link between the preceding and the next, to which it is 

 •closely related. Shell broadest in front, upper contour nearly 

 straight; anterior part of the lower margin evenly arched, posterior 

 margin rather low. Head very short; beak very long, narrow and 

 somewhat incurved; antennules and antennae very small; eye evi- 

 dently larger than the pigment fleck. The post-abdomen is as in 

 P. denticulatus, or a little Ioniser proportionately. The markings, 

 so far as observed, consist of diagonal, faint, numerous and parallel 

 lines posteriorly, and others springing from the anterior margin. 

 There is no tooth behind; the teeth on the post-abdomen are small 

 and not numerous. The upper margin of the shell is not sharp but 

 rounded. Thus this pretty and unique form is clearly distinguished 

 from all its allies although unfortunately only this very imperfect 

 description and schematic figure can be given. Found in Weakly 

 pond, Culbert county, near Florence, Alabama, where with an 

 Alona, Chydorus sphaericus and Scapholeberis, it formed the clado- 

 -ceran fauna of the pool. 



Sp. 10. Pleuroxus unidens, Birge. 



(Plate F. Fig. 15). 



An extreme among these elongated species, the length of body fall- 

 ing little short of double the hight. The dorsal line is very flat 

 and slightly but evenly arched; the lower margin is evenly convex 



