CRETACEOUS. 101 



Genus DISCOPHYLLUM Hall. 



DlSCOPHYLLUM PELTATUM Hall. 



PI. XL VII, figs. 1 and 2. 



Biscophyllum peltatum Hall, 1847, Pal. N Y., Vol. I, p. 277, PI. LXXV, fig. 3. 



Body consisting of a somewhat circular, flattened expansion, composed mainly 

 of radiating fibers, which enlarge as they recede from the center, and terminate in a 

 thickened border. 



This fossil appears to have consisted originally of a semihard circular or oval 

 body, with firmer rays, reaching nearly or quite to the margin. The body becomes 

 thicker and apparently harder toward the margin, and the rays are nearly concealed 

 in its substance. This disk may have been attached by a smaller stipe, proceeding 

 from the lower side, some obscure evidence of such an appendage existing. 



Two specimens of this peculiar fossil have been found, one in 1822 and the 

 other a few years since, showiiig that it is an exceedingly rare form. It is quite 

 unlike any other fossil known in our older strata. 



Position and locality. — This species occurs in the partially metamorphic arena- 

 ceous shales of the Hudson Eiver group, near the nail factory, below Troy, where the 

 only known specimens have been found. (Cabinet of Troy Lyceum; cabinet of 

 Professor Cook.) 



observations. — Through the courtesy of Prof. J. M. Clarke, I have had an 

 opportunity of studying the type specimens described by Professor Hall. 

 There is little to add to his description, except to note the presence of rim-, 

 concentric, undulating- lines that cross both the rays and the interspaces 

 between. There is also indicated on the larger or broken specimen, fig. 2 

 of PI. XLVII, a ring surrounding an area about 2 cm in diameter. The 

 rays, however, cross the ring to the center. This may be the result of the 

 compression of the specimen, so as to bring the opposite side down to a 

 point opposite an aperture of which the ring is the margin, or the ring may 

 be simply a stage in the growth of the specimen. 



It is exceedingly difficult to determine whether D. peltatum is the 

 impression of a medusa. There is no a priori reason why a gelatinous disk 

 should not leave such an impression in the very fine arenaceous silt which 

 now forms the slightly gritty layers embedded in the shales carrying the 

 graptolitic fauna referred to the Trenton terrane. If D. peltabum be con- 

 sidered to be the cast of the impression of a medusa, it might be grouped 

 with Mednsina princeps as an acraspedote medusa. In order to present to 

 the student all the information available, a plate illustrating the two type 

 specimens is introduced (PI. XLVII). 



