12S Annals of ihe Carnf.gie Museum. 



fold and sinus becomes wider and finally forms a broad arch in the 

 front of the valves. 



Plications. — Immediately after the nepionic stage, plications arise 

 on both valves and apparently several are formed simultaneously. On 

 an individual 1.6 mm. in length there are ten plications on the dorsal 

 valve and nine on the ventral. Another individual, of about the 

 same size, has eight on the dorsal and seven on the ventral valve. 

 New plications are added at the sides and are never implanted, nor 

 do they ordinarily increase by bifurcation. In the adult there are from 

 17 to 21. In young stages the plications are sharp and distinct, but in 

 the adult and senile specimens they are more rounded and become 

 broad and faint in front. Two types of plications are found, one in 

 which they are rounded, while in the other they are more angular and 

 have wider spaces between. Fine concentric striae are very numerous. 



Gcrontic Stage. — The senile characters in this species are : hinge 

 width shorter than the width below; strong varices of growth num- 

 erous ; obliterations of the plications toward the front of the shell. 

 One specimen showing these characters is 22 mm. long, 28 mm. 

 wide, and has a width at the hinge of 16.2 mm. There are 19 plica- 

 tions on the ventral valve, all of which become flattened and obscure 

 beyond a growth line at 12 mm. in front of the beak. In nearly all 

 cases the plications are more completely obliterated on the dorsal than 

 on the ventral valve. A larger specimen from East Bethany shows 

 much the same character. It is 27 mm. long, 36.4 mm. wide, and 

 has a width at the hinge of 22 mm. There are 19 plications, all of 

 which become indistinct beyond a growth line 20 mm. in front of the 

 beak. The shell is thickened and there are numerous strong growth 

 lines near the front. 



Other Features : Cardinal Area. — Each valve has a well developed 

 cardinal area which is longitudinally striated. That of the ventral 

 valve is a little wider than the opposite one and has a large triangular 

 to semicircular delthyrium, the apex of which is rounded and en- 

 croaches somewhat upon the ventral beak. The cardinal process 

 nearly fills this opening, but there is, at all stages, plenty of room for 

 the emergence of the pedicle, which is probably functional all through 

 life. The large and strong scar of the pedicle muscle also tends to 

 support this view. No deltidial plates are found at any stage. 



Muscle Scars. — The scars of none of the muscles show distinctly in 

 either of the valves, except in the older stages. Then the scars of the 



