104 



THE AMERICAN NATURALIST 



[Vol. XL 



as it has the same general smoothness ahd evenness of the sides 

 as the delthyrium. 



In those forms of Rhynchotrema capax which continue attached 

 throughout life, the increasing gibbosity makes necessary, even 

 during late maturity, a resorption of the umbo of the pedicle valve. 

 But this resorption becomes very great in senile specimens, as 

 for example in a specimen. No. 1156, the apex of whose pedicle 

 valve has been resorbed anteriorly at least 1.5 mm. (Fig. 11). 

 The smoothness of this opening and the evenness with which it 

 is prolonged out from the interior of the shell show it to result 

 from true resorption and not from breaking. 



b. The pedicle openihg may he partially or completely closed. 

 This is accomplished:— 



(1) By deposits of calcareous matter in the apex of the valve, 

 sometimes forming a callosity. 



Example: in Stropheodonta demissa (Conrad), No. 1914, M. I. 

 T., the delthyrium has been completely closed by growths that 

 extend from either side and meet in the middle; these form two 



convex callosities on the imicr or j>ro\iiii;il side wiiicli meet in 

 the median line. Tlir outside of these irpowths is smooth and 

 also the canliniii inarpn is wautiuLT in the < h-utieuh.tions character- 



A K'.tae.'oiis eallo.itv x.nietimes fori.... i.. the i>.-.heh' cavity, 

 anl cxtnils k i the .lelthyriMm , .ee also Hall and Tlarke, 

 '94a, p. 0). This is se<Mi in Spirijrr annn I naf ii.s- , Conrad), S. 

 granulosils (Conrad), and .S. aiiddciiln.s- (Conrad). 



