Troimdolkptus P'auna at Canandakuja T.akk, N. Y. 125 



1.30 to 1.40. In later neanic stages the length becomes more 

 nearly ecjual to the width and a subcircular form is produced in the 

 adult. The index is then 1.05 to 1.15. In the early neanic stages 

 the width of the hinge is from .75 to .80 of the greatest width 

 below. This proportion decreases rapidly and, in a specimen 3 mm. 

 long the hinge is only .40 to .50 of the width below. This latter 

 ratio is maintained throughout the succeeding growth stages. 



Convexity of l^ah'es — The very young individuals, less than one 

 millimeter in length, have the ventral valve slightly deeper than the 

 opposite one, and specimens three or four mm. in length sometimes 

 have the ventral valve twice as deep as the dorsal. In later stages 

 the two valves become about equal in convexity and individuals above 

 10 or 12 mm. in length, at which stage the ventral valve is nearly flat 

 in front, have the dorsal valve a little deeper than the ventral, revers- 

 ing the relative convexity of the early stages. 



With the first pair of plications a shallow sinus appears on the dor- 

 sal valve. It becomes broad and poorly defined in later neanic stages, 

 and is hardly perceptible on the front of a specimen 12 mm. in length. 



StricB. — The smallest individual in the collection has eight striae 

 on the dorsal valve and seven on the ventral. On the dorsal valve, 

 the middle pair bound the sinus and, since they are the longest, are 

 evidently the first to appear. Outside of these the other six striae 

 seem to have appeared at about the same time. On the ventral valve 

 there is a median striation and three on each side of it. In the next 

 stage observed there are twelve striae on the dorsal valve. Two of 

 the new striae have been added on the lateral margins and two are 

 implanted in the sinus between the first pair. From this stage striiii 

 are added rapidly by both implantation and bifurcation. A shell 

 1.33 mm. long and 1.75 mm. wide has 20 striae, ten of which are 

 ])rimary. At a length of 2.1 mm. there are 25 striae; at 5 mm., 50, 

 and at 11.6, 80. This was the largest specimen on which the striae 

 were sufficiently well i)reserved to be counted. 



Hinge Development. — In the very youngest neanic stages the car- 

 dinal areas are about equal and the beaks diverge at a wide angle. 

 The pedicle opening is shared by both valves, but the opening in the 

 dorsal valve is somewhat modified by the small cardinal process. At 

 a little later stage the cardinal process closes up the opening of the 

 dorsal valve and in adult stages fills a large part of the opening in the 

 ventral area. No traces of a deltidium are preserved on any of the 

 specimens examined. 



