LAMELLIBEANCHIATA FROM THE MIDDLE MARL BEDS. 199 



ARCID^. 



Genus IDONEARCA Conrad. 

 Idonearca medians, n. ap. 



Plato XXVI, I'Mys. 5 and 6. 



This species very closely resembles /. vulgaris in its internal casts, and 

 is very difficult to distinguish in some of its conditions; but the fillings of 

 the cavities of the beaks are very much more elevated, showing that the 

 beaks have been larger and more prominent, and that the cardinal area has 

 ))een very much wider. The inner line of the hinge-plate seems to have been 

 more arched also than in that species. At some localities this species shows 

 the posterior muscular ridges to be extremely large; while the line of the 

 pallial attachment around the posterior end is remarkably strong, and shows 

 strong cicatrices of attachment in the foi-m of short elevated ridges on the 

 cast, crossing a prominent boss, which, of course, represents a deep depres- 

 sion, with still deeper pits, in the shell at this point. This feature, although 

 existing to some degree in nearly all species of the genus as well as in most 

 Areas, is in this one remarkably developed. Exterior and shell so far un- 

 Jcnown. 



Formation and locality. — This species is characteristic so far as yet known 

 of the Middle Marl Beds. The most marked specimens are from Mr. J. S. 

 Cook's marl pits at Tinton Falls, New Jersey. 



Idonearca compressirostra, n. sp. 



Plate XXVI, Figs. 15 and 16. 



Cast of small size, subquadrangular in outline and only moderately 

 -convex; beaks small, situated near the anterior end, but very slightly ele- 

 vated and approximate; anterior end of the shell broadest, the margin 

 abruptly declining from the beaks to near the basal line; basal line moder- 

 ately arcuate; posterior end obliquely truncate, and narrower than the 

 anterior end; cardinal line highly arched between the beaks and gently 

 ■declining posteriorly. Muscular scars faintly marked, the anterior one 



