DESCRIPTION OF SPECIES. 19 



B has a width of 4 inches and an estimated length of 24 inches. Frond 

 C has a width of 6.4 inches and an estimated length of 40 inches. The 

 widths were measured and the lengths estimated from fragments large 

 enough to give a good idea of the true size of the frond. It is necessary 

 to estimate the lengths, as the plants are never, when of large size, com- 

 plete. I have, however, seen fragments so nearly representing the entire 

 length that I can confirm these estimates and measurements of Professor 

 Rogers. The enormous number and wide diffusion of the specimens of this 

 plant, and the different stages of growth which are preserved, afford a good 

 deal of variation both in the size and shape of the impressions, so that 

 sometimes one is tempted to suppose that he has a new species before him. 

 I have, however, found in all cases that the nervation remains the same. I 

 have obtained this plant in all stages of growth, from the very young form 

 to the fully-grown leaf, and in all degrees of perfection of preservation. 

 In many cases the shale is so fine-grained, and the plant presented in so 

 many aspects from the maceration which it has undergone, that I have 

 been enabled to make a very satisfactory study of it. 



The young plants represented in Plate II, Fig. 3, and Plate III, Fig. 2, 

 are seen to assume a rather broadly elliptical form, thus differing greatly 

 from the more fully grown plants. A marked character of the young frond 

 is the great and sudden attenuation of the midrib, which occurs about mid- 

 way its length. Both Professors Rogers and Bunbury have called atten- 

 tion to the fact that two forms are quite common in the more fully grown 

 fronds. One form is elongate, and gradually narrows at the summit, giving 

 the frond an oblong-lanceolate outline. This I have represented in Plate 

 IV, Fig. 3, which is a much reduced outline of a full-grown leaf. The other 

 form has the summit more bluntly rounded off, and possesses an oblong 

 spatulate shape for the whole leaf. This is represented much reduced in 

 Plate IV, Fig. 4. Plate V, Figs. 1 to 3, represent one of the obtuse fronds of 

 natural size, as made out from a nearly complete specimen. Plate III, Figs. 

 1,1a, represent one of the smaller acute fronds of natural size, and Plate III, 

 Fig. 3, gives the summit of a larger frond of the same shape, also of natural 

 size. Plate IV, Fig. 2, represents a rather unusual shape of the plant, where 

 the length is great in proportion to the width, and in which the undulations 



