MOLLUSCA. 395 



ing downward at a sharp angle to the main portion of the tooth, 

 the median two-fifths of the hinge occupied by short, vertical 

 teeth about 10 or 12 in number. The muscular scars on the 

 interior of the shell well-defined, the posterior one bordered along 

 its anterior margin by a narrow, elevated septum ; pallial line dis- 

 tinct, the internal surface of the shell above the pallial line marked 

 by more or less indistinct radiating lines. 



The internal casts ventricose and very oblique, with the beaks 

 elevated, large and distant, and with a strongly subangular 

 umbonal ridge extending obliquely backward from the beak to the 

 postero-basal extremity. The postero-dorsal slope short and 

 more or less abrupt, the junction of the valves usually strongly 

 keeled when perfect, though this keel is often broken away. The 

 cardinal line curved. The gash in the postero-dorsal slope formed 

 by the septum in front of the posterior muscular impression is 

 deep and often wide in old individuals, extending from the middle 

 of the height oi the shell to fully two-thirds the distance between 

 the margin and the beaks. Anterior muscular scar more or less 

 distinct, body of the cast often radially striated. 



Remarks. The description of this species, given above, is 

 drawn from finely preserved individuals in the Marshalltown 

 marl. Among the many perfectly preserved shells of the species 

 in this formation near Swedesboro, considerable variation is ex- 

 hibited, especially in the abruptness of the postero-dorsal slope. 

 In some individuals this slope is nearly vertical while in others it 

 may be at an angle of nearly 45 to the plane of the valve, these 

 extremes giving to the shells quite different aspects. ^ A rather 

 abrupt, but not quite vertical slope is the commoner, but all in- 

 termediate conditions between the two extremes occur. Another 

 variable character is the surface markings of the shell, some in- 

 dividuals being quite rugose in appearance because of the strong 

 concentric lines of growth, while others are much smoother; 

 the radiating costae are also variable, in most cases they can 

 scarcely be detected and in none are they really conspicuous. 



The casts which Whitfield has illustrated as Idonearca vulgaris 

 are apparently members of this species, but his specimens seem 

 to have been lost or destroyed so that no direct comparison can 



