256 



sliglitly straigliteued towards the front. Beak small, acute, and but slightly 

 extended beyond the hinge area, of which it is impossible to determine the 

 exact size and shape, since it is iuv^ariably concealed by the rock. 



Dorsal valve Hat, or curving gently from the beak to the front and sides, 

 with a well defined sinus along the median line, corresponding with the fold of 

 the ventral valve, and which, narrow at the beak, increases gradually in width, 

 becoming moderately deep at the front. A rounded, prominent plication occu- 

 pies the bottom of the sinus, and the margins curve up abruptly to form a 

 large plication on each side. 



There are four or five, seldom six, broad, rounded, plications on each side of 

 the fold and sinus, separated by depressions of a similar character. There is 

 a great variation in the size of the plications, which are much larger in some 

 specimens than iu others. Those of the ventral valve are, however, always 

 narrower than the intervening depressions ; while on the dorsal valve the de- 

 pressions are the narrower. The plications extend nearly directly from the 

 beak to the margins, arching somewhat strongly along the top near the fold 

 in the ventral valve, but less and less so, becoming smaller, and less distinct 

 towards the cardinal angles, which are sometimes flattened or even slightly 

 reflected, and are smooth in both the valves. There are usually several lines of 

 growth. The entire surface is traversed by very fine radiating raised lines, which 

 rise at regular intervals into minute, hollow spines, with elongated bases, the 

 inner surface of the test showing their position as slight depressions. The 

 minute surface markings are seldom seen on the specimens from the sandstone, 

 which, even when best preserved, show only the bases of the spines, and those 

 very indistinctly. But several moulds of valves, obtained from the underlying 

 yellow shales, have the impressions of the raised lines and the spines well 

 preserved. 



The impressions of the hinge teeth are shown in the moulds of the ventral 

 valve, and in the interior moulds of the dorsal valve the impressions of the 

 processes are partially preserved ; but on account of the coarseness and friable 

 character of the sandstone in which they occur, we cannot depend upon them 

 as being at all perfect. The cardinal process is somewhat angular behind, and 

 the socket plates are rather broad at the base, but become narrow along the 

 top. The septum is short and low. 



A ventral valve of ordinary size measures 11 m.m. in length, 1.3 m. m. in 

 width and about 3 m. m. in height, but specimens are often found much larger, 

 one being 16 m. m. long, 25 m. m. broad and about 5 m. m. high. 



There would be no difticultj in separating the Erere specimens 

 of Vitulina from the small forms of V. jmstidosa, Hall, described 

 and figured in Vol. IV of the Pal. of New York. But since Prof. 

 Hall wrote the description of this species, he has obtained a great 

 number of specimens from other localities than the first, many of 

 Avliich differ much from those first described, frequently being 



