2o2 



lateral margins, nearly straight, or gently convex, extending forward one-half 

 the length of the valve or more, and then forming, with the anterior margin, a 

 very regular curve around the front. 



Ventral valve moderately convex, usually with the greatest elevation just 

 behind the middle, whence, with a regular curve, it slopes more or less rapidly 

 to the front. Towards the beak it carves quite abruptly, while across the 

 middle the curvature is moderately strong and regular. In a few instances, 

 however, the valve is most prominent in the middle. The sides curve slightly 

 inwards towards the cardinal angles, which are flattened or slightly reflected, 

 making the curve thence to the center of the valve slightly sigmoidal. Beak 

 much depressed, with the small, acute apex scarcely projecting beyond the 

 hinge area. Septum small and about one-fourth as long as the valve. The 

 number and length of the spines is unknown. The impressions of a portion 

 of two of them are preserved, with the exterior mould of one specimen ; 

 these are slender, and diverge obliquely outwards from the cardinal margin, 

 their length being about one-fourth that of the valve. 



Dorsal valve varying from moderately to very slightly concave, most de- 

 pressed towards the front, and rising gradually towards the hinge line, along 

 which, and at the cardinal angles, the valve is often flattened. 



The raised lines, with which the valve is ornamented, are small and rounded, 



and increase very slightly in size towards the front, where they number from 



15 to 23 or more. But from the imperfect preservation of the moulds in the 



sandstone, the Hues are always obliterated on the sides and the posterior part 



'of the test, while, in many cases, the whole test is thus rendered smooth. 



One specimen of ordinary average size measures 7 m. m. in length, 9.5 m. m. 

 in breadth, and 2 m. m. in height. 



This species, in its typical forms, seems to be related to Ch. ar- 

 mata, Bouch., Dev. Inf., Boulonuais, France, with specimens of 

 "which I have very carefully compared it. The Brazilian species is, 

 however, a variable one, and the radiating lines are always coarser, 

 and, when well preserved, are more prominent than in C. armata. 

 The ventral valve of C. armata is also generally more elevated. 



The specimens of C. Herbert- SmWiii, on Avhich the ornamenta- 

 tion is well preserved, resemble somewhat in general appearance 

 both C. dejlecta, Hall, and C. laticosta, Hall, of the Corniferons and 

 Hamilton groups; but a close examination shows that, in shape, the 

 Erere species differs entirely from those of New York. The beak 

 of the foraier species is always much depressed, while in the latter 

 it is prominent, the whole nmbonal region of the test being much 

 elevated. 



