Foerste.] 
300 
[May 1, 
to find on the anterior border, just beneath the geniculating ridge, 
a low wrinkle which will be the geniculating ridge of the next sea- 
son. This method of growth often seems to continue through life, 
but in the Clinton Group of Ohio and in the Niagara of Indiana, 
specimens are found in which this process of the retraction of the 
anterior border to the plane of the original shell has ceased and the 
anterior border continues growth without further wrinkling. A spec- 
imen from Huffman’s Quarry is 22 mm. long as far as the genic- 
ulating ridge, and has an abruptly descending, perfectly unwrinkled 
border 23 mm. long. The shell is 38 mm. broad. 
Strophomena patenta, Hall. 
(PLATE V, FIG. 22.) 
This species was described by Hall from the Clinton Group of 
New York, and by Hall and Whitfield from the Clinton at Da}Ton, 
Ohio. It is also found at Fair Haven and Fauver’s Quarry, Ohio, 
and at Hanover, Indiana. A fragment from Collinsville, Alabama, 
offering no distinguishing features is also placed here. The genic- 
ulation of the New York forms amounts only to a well developed 
but not strong double curvature of the shell. In the Alabama spec- 
imen this double curvature is even less marked than in the New 
York specimens. In specimens from Fair Haven and Fauver’s 
Quarry, and in those from the Soldiers’ Home Quarries, the curva- 
ture is about the same as that of the New York specimens. At 
Soldier’s Home is also found another form in which the double 
curvature is very pronounced and the reversed curvature anteriorly 
amounts almost to geniculation. 
The variation in the amount of the anterior curvature can be in 
part accounted for by the fact that the more strongly curved shells 
are more apt to be the dorsal valves, and those less curved the ven- 
tral, but a little study will show that the more strongly curved shells 
have, as a rule, slightly finer striations, and consist of both ventral 
and dorsal valves, while the less curved shells have somewhat 
coarser striae and agree more readily with the New York specimens, 
taking an intermediate position as to the coarseness of the striae. 
This shell begins its existence with the ventral valve convex and 
the dorsal concave. Later the anterior and lateral margins of the 
ventral valve become concave or turned up, and that of the dorsal 
valve convex or turned down. Since this is likewise true of Stro- 
phodonta striata and only the exterior of these specimens is found 
