110 TWELFTH ANNUAL REPORT. 



Sp. 7. Pleuroxus clenticiilatus, Birg^e. 

 (Plate a. Figs. 12—13.) 



Resembling very closely P. aduiictus, which, however, has a 

 broader post-abdomeu than the ordinary P. denticulatus. The 

 posterior angle of the shell is armed with from one to four (gener- 

 ally three) teeth. The beak is very long. 



The character most emphasized by Birge is a series of teeth along 

 the anterior margin of the valves. The same thing is found in P. 

 procurvus, as I have repeatedly satisfied myself. In certain posi- 

 tions these teeth do not show, or the smaller teeth on the lower 

 margin only appear. P. adunctus, as figured by Schoedler, has 

 similar teeth on the lower margin, and very likely has them anteri- 

 orly. The edges of the valves are heavily fringed with pectinate 

 setae. The male has a shorter beak and the post-abdomen simply 

 rounded without the peculiar modification seen in P. adunctus. 



There seem to be two varieties in Minnesota both of which have 

 the characteristic irregular striations of the shell, which radiate from 

 an irregularly marked or unmarked area in the center toward the 

 edges; both have the toothed posterior angle and the serrated post- 

 erior angle and the serrated anterior margin. But the common 

 form is much longer, with the dorsal margin less convex and the 

 beak shorter. The robust form has a larger pigment fleck, while 

 the post-abdomen is shorter and more robust, resembling more 

 nearly Schoedler's figures of the abdomen of P. adunctus. There is 

 another variation or abnormalitj^ in which the lower margin is 

 •quite concave. The resemblance to P. procurvus is remarkable in 

 some phases. 



I have collected this species in Blount springs, Ala., in the St. 

 Croix river, and at various intermediate points, as well as very often 

 in Minnesota. 



Sp. 8. Pleuroxus hamatus, Birge. 



(Plate H. Fig. 1.) 



This species is smaller than those of the preceding group and 

 forms a transition to the two next to be described in the greater 

 elongation of the shell, which is, however, higher and more strongly 

 arched. The head and beak are much as in P. denticulatus. The 

 lower margin is concave posterior to the middle and slightly convex 

 at the posterior angle, which is unarmed. The lower margin is 

 hairy. The markings are as in P. denticulatus, but, in addition, 



