FOSSILS OF THE NIAGARA GROUP. 379 



dinal extremities are rounded and the area more or less distinctly defined. 

 Some remains of surface stride are sometimes distinguishable on the cast, 

 and this marking is often well preserved in the matrix from which the 

 shell has been dissolved. Sometimes the margins of the valves, or their 

 impression in the cast, are undulated, and there are obscure remains of 

 broad low plications, which usually extend but a short distance, though 

 sometimes continuing nearly to the beak. The latter forms may perhaps 

 be regarded as 8. plicatella proper ; such specimens are very gibbous, 

 with a high area and broad deep sinus in the ventral valve, while they 

 differ in form from the simply striated specimens. 



The peculiarity noticed in the smooth or finely striated species is the 

 presence of distinct lamellse in the dorsal valve (as shown in fig. 9 of 

 Plate xiii) diverging from the apex and presenting all the characters 

 of the dental lamellse of the ventral valve. These marks upon the cast 

 are not simply sharp cut depressions, but the edges of distinct thin plates, 

 which are joined to the inside of the shell, sometimes for half its length. 

 A specimen, fortunately broken, shows the interior of a shell without 

 filling, and these dorsal lamellaB are seen extending downwards half way 

 to the base, and uniting with the shell precisely as the dental lamellaa 

 of the ventral valve. These lamellae are divided near their origin, and 

 give off the crura from which the spires have continued.* 



Specimens of this character are rhomboidal, gibbous, with distinct 

 mesial sinus and fold ; small specimens like the one figured, are more 

 common than larger ones, though they are sometimes found of much 

 larger size, and assuming a transversely oval form. From all the obser- 

 vations made, it appears as though the dorsal lamellae were much stronger 

 in the young shell, and that they become partially absorbed or almost 

 entirely disappear in the older shells. In specimens regarded as the 

 same species from Indiana and New York, the evidence of lamellae is 

 confined to the apex of the valve, and is never observed to extend 

 towards the front of the shell. 



Formation and Locality. — This species is common in limestone of the 

 Niagara age, at Racine, Wauwatosa and elsewhere in Wisconsin, and at 

 Bridgeport, Illinois. 



* These features are more fully shown in flg. 6 of Plate sxv. 



