Troi'idolki'tus Fauna at Canandaigua Lake, N. V. (I."> 



Convexity of Valves. — Immediately at the end of the neiMonic stage 

 the dorsal valve becomes slightly concave, and remains concave or 

 flat throughout the remaining stages of its development. The pedicle 

 valve is convex in all stages, but never strongly so, producing a nearly 

 flat shell. 



Slrice — Just at the anterior edge of the smooth nepionic shell the 

 strice begin. They are introduced in pairs on the dorsal valve while 

 the pedicle valve has an unpaired median striation. A shell .75 mm. 

 long and i mm. in width has seven striae on the ventral valve and 

 eight on the dorsal. Before the shell is i mm. in length the older, 

 that is, the middle striae, have bifurcated, and, from that time, the 

 increase of striae is rapid and new ones are added by both bifurcations 

 and imi)lantation. A shell 1.8 mm. long has 19 stride. One 2.8 

 mm. long has 45, one 8 mm. in length has 140, and one 16 mm. 

 long has 200. In neanic stages the extended cardinal angles are 

 smooth, even after the striae become numerous in front. In adult 

 shells, the stride cover almost the entire surface. Very fine concen- 

 tric lines cross the striae. Near the umbo and occasionally all over 

 the shell, are faint concentric undulations. They are especially 

 strong in young shells and dorsal valves. 



Crenulations. — On the smallest shell, in which the hinge is pre- 

 served .2 X 2.5 mm., the crenulations extend .6 of the distance from 

 the beak to the cardinal extremity (not an alate shell). On a larger 

 shell, 5.3 mm. wide at the hinge, .8 of the distance is crenulated, and 

 in all individuals larger than that, the crenulations extended the 

 whole width of the shell. 



Pedicle Opening. — The material does not illustrate this part of the 

 development well. No very young specimens retaining both valves 

 were found, and the young ventral valves are all broken at the beak. 

 An adolescent individual, 5.6 x 8.4 mm. shows a flat deltidium, whose 

 limits can hardly be distinguished, and a relatively large i)edicle 

 opening just behind the beak. In adult specimens there is a more or 

 less convex deltidium, often not well defined at the sides, and the 

 pedicle opening is very small and pushed forward on to the beak. 



Muscle Scars. — The muscle scars of young specimens are not 

 strongly marked in either valve. In the ventral valve of the adult, 

 the diductors leave elongate, flabelliform scars. Between them, lying 

 on either side of the low median septum, are the two long oval scars 

 of the adductors. Diagonal lines divide each scar, marking the limits 

 of the anterior and posterior elements (Fig. 12). 



