MOLLUSCA. 399 



because of the crudeness of the drawing. These original speci- 

 mens, however, show conclusively that the species is the one 

 which sometimes occurs so commonly in the Hornerstown marl, 

 to which Whitfield gave the name Idonearca medians. The 

 casts referred to Idonearca vulgaris by Whitfield are quite differ- 

 ent and probably should be referred to C. tippana. The species 

 resembles in some respects the internal casts of C. tippama, but 

 it is more oblique with the beaks longer and more pr6minent, 

 with the posterior muscular ridge usually more prominent, and 

 with the pallial line much more conspicuous and marked by 

 the strong transverse ridges. The species is known only in 

 the form of internal casts so that the external characters 

 of the two species cannot be compared. The beaks of C. 

 vulgaris, however, were undoubtedly more widely separated 

 and the cardinal areas wider. The species, so far as it has 

 been observed in the recent collections, is a characteristic form 

 of the Hornerstown marl horizon, and it is possibly a genetic 

 successor of C. tippana of the earlier faunas. 



Formation and locality. — Hornerstown marl, J. S. Cook's pits, 

 Tinton Falls (Whitfield), near Hornerstown (152). 



Geographic distribution. — New Jersey. 



Cucullaea compressi rostra (Whitfield). 



Plate XXXII., Figs. 3-4. 



1886. Idonearca compressirostra Whitf., Pal. N. J., vol. i, 

 (Monog. U. S. G. S., vol. 9), p. 199, pi. 26, figs. 15-16. 



Description. — Internal casts subtriangular in form, small, the 

 dimensions of a nearly perfect individual being: length 26 mm., 

 height 21.5 mm., thickness 16 mm. Anterior margin curving 

 regularly downward and backward from the anterior extremity 

 of the hinge-line into the moderately curved basal margin, 

 postero-basal extremity rounding somewhat abruptly into the 

 gently convex, obliquely subtruncate posterior margin. Beaks 

 of moderate size, moderately elevated and approximate. Um- 

 bonal ridge rounded, postero-dorsal slope making an angle of 

 about 45° with the plane of the valve. Muscular scars faintly 



