MOLLUSCA. 425 



vated above the hinge-line. The anterior margin sloping forward 

 from the beak and rounding gradually into the broadly rounded 

 "basal margin, posterior margin broadly rounded and meeting 

 the hinge-line in an obtuse angle. Surface of the shell marked 

 by more or less rounded or subangular, concentric undulations, 

 which are often somewhat irregular in the strength of their de- 

 velopment and in their distances apart. In addition to the un- 

 dulations the surface of the shell is marked by fine concentric 

 striae separated by intervals of I mm. or less. 



Remarks. This species is the common member of the genus 

 in the Merchantville clay-marl, and includes, without doubt, the 

 two specimens illustrated by Whitfield as /. sagensis. In Owen's 

 original description of I. sagensis, which occurs typically in the 

 western faunas, it is distinctly stated that the shell is not marked 

 by concentric striations, while the New Jersey examples always 

 have these lines when the surface markings can be seen ; further- 

 more the valves of the eastern form are usually less strongly con- 

 vex than those of the western specimens. The concentric striae 

 are usually not recognizable upon the internal casts, although 

 they may sometimes be detected, but they can almost always be 

 seen upon the impressions of the outside of the shells. These 

 striae in most cases seem to be slightly raised lines, but in one 

 specimen from the Cliffwood clay they are apparently impressed. 

 One large example from Lenola with a length of 135 mm., has 

 a broad, smooth area without undulations around the free mar- 

 gins of the shell, a character which is sometimes seen in other 

 species of the genus and which doubtless represents a senile stage 

 of growth. Among the various specimens examined, there is 

 considerable variation in the width of the spaces between the con- 

 centric undulations of the shell, and in some specimens these un- 

 dulations bifurcate anteriorly and more rarely posteriorly. 



Formation and locality. Cliffwood clay, Cliffwood Point 

 (105), near Matawan (189) ; Merchantville clay-marl, near 

 Matawan (ioo 3 , ioo 4 , 101), near Jamesburg (139, 141), Lenola 

 (163); Marshalltown clay-marl, near Swedesboro (177). 



Geographic distribution. New Jersey, Mississippi, Arkansas, 

 Texas. 



