PAROPSONEMA CRYPTOPHYA ' 173 



opposite faces, the space intervening had not more than the 

 thickness of a piece of blotting paper and had been filled, not 

 with matrix or any ancient deposit, but with a comparativel.y 

 recent, discontinuous deposition of infiltrated, amorphous cal- 

 cite of the same character as that elsewhere observed on more 

 exposed partings of the matrix. In another specimen this in- 

 terval is greater, though variable, and is seen to be filled with 

 the sandy matrix. It is to be assumed that such disk-shaped 

 bodies in rocks of this character under no little vertical pressure 

 from superjacent sediments, must have, unless highly resistant, 

 been compressed to the almost complete extinction of the interior 

 space; and, on investigation of these structures, it seems that 

 the original matter of the fossils, so far from being capable of 

 withstanding high pressure, probably yielded to very slight 

 strains. 



One of the two sides is smooth, that is to say, devoid of regular 

 structure or intimate detail. Its surface is however wrinkled 

 and puckered. Where this character is best manifested it is 

 clear that the largest of the wrinkles as well as the vast number 

 of very minute ones are the outcome of compression; the former 

 are pinched up into one or two prominent folds, the others lie 

 over the surface principally of the median portion. This median 

 part of the fossil evinces by these indexes the greatest compres- 

 sion. More regularity marks the series of wrinkles which depart 

 from the central area toward the periphery, ramifying, inosculat- 

 ing and spreading outward but becoming extinct before the 

 actual margin of the disk is reached. In some cases traces of 

 finer, direct radial striae are visible over the smooth peripheral 

 border, but these are not always clear. Such is the aspect of the 

 surface which for convenience and perhaps with propriety we 

 may term the ventral ; but it is the aspect shown by the removal 

 from this surface of the opposite side of the disk, and doubtless 

 its contour is that of the interior side of this ventral shell. Let 

 us reverse, therefore, the contour as described and we shall have 

 the aspect of the exterior ventral surface, on which there will 

 be the casual wrinkles as before, but the radiating and true 

 structural features will be anastomosing grooves and channels, 



