199 PALJIONTOLOGY OF NEW-YORK. 



8|»irif'ci-a raricosta. 



PLATE XXVII & PLATE XXX. 



DtlthyrU raricotta : CoNRjm, Journ. Acad. Nat. Sci. of Philadelphia, Vol. viii, p. 262, pi. 14, f. 18. 1842. 

 Vellhyrit mdulalui : Vasuxeu, Geol. Report Third District New-York, p. 132, f. 3. 1842. 



Shell subquadrate, semicircular or ovate, gibbous : hinge-line equalling 

 the width of the shell or often less ; cardinal extremities rounded. 

 Surface strongly plicated. , 



Ventral valve most gibbous in the upper half, and sloping abruptly to 

 the cardinal angles, which are rarely a little extended and subauricu- 

 late : beak much elevated, and always incurved over the area which 

 is variable in elevation, sometimes being barely perceptible, while 

 in others it has a width of from one to two lines, and is marked lon- 

 gitudinally by a few strong striae. The mesial sinus is a broad rounded 

 dejiression, and reaches with the adjacent plications quite to the apex. 

 Dorsal valve gibbous, most convex in the middle, flattened or a little 

 concave towards the cardinal angles. The mesial fold is very promi- 

 nent and rounded or a little flattened in the middle, regularly arcuate, 

 and forming the small beak which is arched over the linear area. 

 Surface marked by two to four strongly elevated rounded or subangular 

 ribs on each side of the mesial fold and sinus. In one specimen, I have 

 seen a fifth plication towards the cardinal angle. The entire shell is 

 covered by strong lamellose or imbricating concentric stria3, which 

 are undulated upon the ribs and the intermediate depressions. These 

 concentric lines are sometimes quite regularly equidistant, but often 

 crowded and irregular in their distribution, and more or less promi- 

 nent at their edges. There are a series of fine closely arranged radia- 

 ting striae crossing the concentric lamellae, and giving in very perfect 

 shells a fimbriate aspect. 



The surface is usually more or less worn, and only the stronger con- 

 centric lamellae are visible ; and even these are often partially or entirely 

 obliterated. In some specimens, where the shell is well preserved, the 

 plications are rounded and not very prominent ; while they become more 



