CONTRIBUTIONS TO PALEONTOLOGY. 77 



"The singular and graceful forms first noticed in tKe Cauda-galli grit 

 " reappear in this group, and are common to many localities. The forms 

 " are better defined in this rock than in the lower one, and the parts are 

 " all united or confluent; showing, in other words, a continuous surface, 

 " and not one of detached parts either real or apparent, as in those of the 

 " other rock."* 



Similar forms occur in the Chemung group of New- York and 

 Ohio, all presenting the peculiar character of surface, viz. a cur- 

 vilinear outer margin, sometimes a little thickened, to which all 

 the parts are directed from a common centre, in curving lines ; for 

 however even may be the surface of the frond, there are either 

 thickened portions or curving strise, which show the direction and 

 mode of growth. 



I have had in my collections, for a long time,f specimens which 

 illustrate the mode of growth, and to some extent the perfect 

 form of these peculiar fossils in at least one species. 



The form has been that of a spiral frond, growing upward's from 

 a small base, and gradually expanding in its successive volutions. 

 The axis is sometimes, and perhaps always, thickened ; and por- 

 tions of this, when torn away with broken parts of the frond, 

 give the aspect of a" stem," as spoken of and illustrated by Mr. 

 Vanuxem. I have ascertained this mode of growth and form of 

 the fossil, by separating successive laminse of the shale, and 

 tracing the continuation of the same frond upwards as it ap- 

 pears in the enlarging discs upon the successive surfaces. In this 

 manner they have been traced from where the diameter is less than 

 one inch, and apparently near their origin ; and thence through 

 the gradually expanding volutions till they have reached the dia- 

 meter of several inches, the spaces between the volutions being 

 several times greater than the thickness of the frond. The volu- 

 tions and the form of the disc often, and perhaps usually, continue 

 very regular till the turns have reached a diameter of four or five 

 inches ; while the larger fronds not unfrequently present irregu- 

 larities and distortions, both from unequal growth and from ac- 

 cident, evidently having been very flexible and easily disturbed. 



From the character of the fragments occurring in association 

 with the regular spiral forms, it would appear that the upper 

 portion of the frond often grows more freely, or more rapidly 

 ascending; having the appearance of a broad band irregularly 

 contracted into festoons. 



* Geological Report of Third District N.Y. Survey, p. 156; figures pp. IGO & 177. 

 t Since 1838. 



