394 CRETACEOUS PALEONTOLOGY. 



Cucullaea tippana Conrad. 

 Plate XXXI., Figs. S-io; Plate XXXII., Pigs. 1-2. 



1858. Cucullaa tippana Con., Jour. Acad. Nat. Sci. Phil., 2d 



sen, vol. 3, p. 328, pi. 35, fig. i. 

 1861. Cucullaea tippanu Gabb, Synop. Moll. Cret. Form., p. 174 



(118). 

 1864. Cucnllcca tippana Meek, Check List Inv. Foss. N. A., 



Cret and Jur., p. 8. 

 1886. Idonearca tippana Whitf., Pal. N. J., vol. i (Monog. U. 



S. G. S., vol. 9), p. 95, pi. 12, figs. 19-21. 

 1886. Idonearca vulgaris Whitf., Pal. N. J., vol. i (Monog. 



U. S. G. S., vol. 9), p. 95, pi. 12, figs. 19-21. 

 1905. CucuUcca tippana Johns., Proc. Acad. Xat. Sci. Phil., 



1905, p. 8. 



Description. — Shell attaining a length of 64 mm., a height of 

 51 mm., length of hinge-line 41 mm. and convexity of one valve 

 20 mm., in a large individual. Shell very oblique, subtriangular 

 or subtrapezoidal in outline; umbones promineiit, the beaks 

 rather small, incurved, situated at about the middle of the hinge- 

 line; cardinal areas concave, marked by distinct impressed lines 

 about I mm. apart, which diverge fro^m beneath the beaks. An- 

 terior margin of the shell curving regularly from the anterior 

 extremity of the hinge-line into the basal mai;gin, which is nearly 

 straight or slightly convex posteriorly ; postero-basal extremity 

 rather sharply rounded or subangular; posterior margin obliquely 

 truncate. Surface of the shell most strongly convex along the 

 subangular umbonal ridge, the postero-dorsal slope abrupt, in 

 some individuals being almost vertical. Surface marked by con- 

 spicuous, more or less irregular lines of growth, and by fine 

 radiating costae, which are frequently nearly or quite obsolete. 

 Hinge-plate broad at either end, much narrower under the beaks, 

 with from three to five transversely striate horizontal 'teeth in 

 front and behind, which are subparallel with the hinge-line and 

 are progressively more elongate from below upward, the inner 

 extremities of one or two of the higher more elongate teeth bend- 



