STATE GEOLOGIST. Ill 



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

 men is widest in the middle and almost exactly as in P. denticula- 

 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 Alabama, My notes 

 contain no reference to the minute striations, which could perhaps 

 be hardly seen with the instrument emplo3'ed. The process of the 

 iabrum 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 aflflnis. (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 

 <jlosely 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 antennas 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, 

 «o 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 althongh 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 sphasricus 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 



