536 MESOZOIC FLOKAS OF UNITED STATES. 



expansa iiiven in Monograph X\', pi. cxxxv, figs. 15, IS, 22. l)ut here 

 the ni:il(>rial is clay. The clay retaining the shape of the cone has 

 undergone some modification and induration not found in that which 

 embeds it, for the mud cones may he picked out of the matrix retain- 

 ing their shape. The pits are clearly not so numerous as the scales 

 of the cones were originally. The precise mode of fossilization is not 

 evident. It would seem that the ripened cones, retaining a few widely 

 divergent and persistent scales, fell to the l)ottom and were l)uried in 

 the accumulating mud sediment. Then the surrounding mud was 

 pressed in Ijetween the scales and took the form of the cone. After- 

 wards the scales decayed and left pits to represent them. These pits 

 sometimes give very well the shape of the vanished scale. They indi- 

 cate that the ends of the scales had Ijroad peltate forms, and that they, 

 toward their insertion on the axis, were greatly attenuated. 



The clay retaining the form of the cones seems to have been indu- 

 rated, as stated before, so that the fossils can be separated from the 

 surrounding mud. Probably this was caused l)y silica deposited from 

 solution. The silica may have l)een lirought into solution by the action 

 on the surrounding rock material of the vegetable acids produced in 

 the decay of the material of the cones. I have often observed indica- 

 tions of such action in other cases. For example, limbs of trees, once 

 embedded, have been found now represented only by hollows or molds 

 which take their form. The walls of the molds were impregnated with 

 silica deposited from solution and were often so strengthened that the 

 molds were kept open. 



PL CIX, Fig. 12, represents one of the smallest, but not the 

 smallest, of the cones, and Fig. 13 one of the largest. Both of these 

 occur in Mr. Hatcher's collection and are deposited in the National 

 ^luseum. As will be seen by the above account, there are in all the 

 collections 98 specimens of these cones. 



As above stated, Coffin's old engine bank, Muirkirk, also furnished 

 Mr. Bibbins a single doubtful specimen of C. acutum. The label is 

 marked "M. G. S., 9774." 



In the collections of the Maryland Survey from the Muirkii-k local- 

 ity are several specimens of so-called white ore — that is, carljonate of 

 iron — which show rootlets that can l^e identified with no species. 



