Notes on Radnlcc. 



87 



base of attachment longer than wide, the lower outer angle 

 expanded; the reflexion is very broad and bicuspid, the 

 inner cusp being very large and semi-bifid, and the inner cusp 

 is small and narrow, and there are two very small denticles 

 on the edge of the upper part of the outer cusp ; the fifth 

 lateral is somewhat narrower, and the inner cusp is split to 

 form a large central cusp, with a small side cusp on each side ; 

 the marginal teeth are very narrow, with a long, narrow re- 

 flexion, which is distally tricuspid, the central cusp being 

 the larger; the outer edge of the reflexion is serrated by 

 about five small denticles. The bifurcation of the inner cusp 

 of the lateral teeth is difficult to see, but it has been distinctly 

 observed on a number of teeth. The cutting points are very 

 well developed, but are also difficult to see clearly. Ninety 

 rows of teeth were counted, and in one individual 20-1-20 

 teeth were found. 



Ancylus tardus Say. 



The radula of this species does not seem to differ essen- 

 tially from that of A. rivularis. 



Physa hetkrostropha Say. Plate V, C. 



Jaw in one piece, arched, striated, provided with a central 

 fibrous projection from the superior surface ; ends rounded 

 (C, c). 



Radula formula ff -f- - 9 T 5 - -f- 2-5-2- A \~ ~t~ ~f* ft (1 90-1- 190) ; cen- 

 tral tooth more or less quadrate, the lower outer corners being 

 very much attenuated ; cusp attached at base, 9-quadrate, 

 five dentals being long and narrow, and two on each side 

 small and blunt; laterals on two alternate series, the first or 

 primary teeth being large, obliquely inclined, comb-like, and 

 13-dentate, six denticles being long and pointed and seven 

 short and wide. The secondary teeth are long and narrow, 

 with a blunt cusp. These latter are very difficult to see, on 

 account of their small size ; the central teeth are also difficult 

 to make out. Mr. W. H. Dall gives a very good figure of the 

 central tooth (Am. Lyc. Nat. Hist., N. Y., Vol. IX, pi. ii, figs. 

 8, 10), and a number of primary and secondary teeth. His 



7 



