218 REPORT ON THE STATE CABINET. 



3. THE CEBTTEAL DISC. 



In several of the species having four simple stipes, in one species with 

 eight, and in another with a larger number of simple stipes proceeding 

 from a common centre, we find their bases united by a thickened corneous 

 expansion of the same substance as the body of the Graptolite. This 

 appears to be composed of two laminse, which, at least, in the central 

 portions are not conjoined, and the space is probably occupied by some 

 softer portion of the animal body (Plate iii, and figures 3, 4 and 6, pages 

 173, 174 and 175). 



The substance of the disc sometimes extends along the margins of the 

 stipes, producing an alation, which may be observed in a slight degree in 

 the figures referred to ; while in one species {G. alatus), the extension is 

 twice as great as the width of the disc, extending an inch beyond the axil. 



This arrangement of the parts of the body seems obviously adapted to 

 give strength and support to the bases of the stipes ; but beyond this 

 it probably serves other purposes of the animal economy. In several 

 specimens of G. hicornis there is a disc or bulb at the base of the stipe, 

 which, spreading between the two oblique curving processes, envelopes, 

 in the compressed condition of the specimens, some of the celluliferous 

 part near its base. In other specimens we have a crescent-shaped exten- 

 sion, as if the disc were in process of development, or perhaps of absorp- 

 tion. Much the larger proportion of the specimens of this species, 

 however, are destitute of disc or bulb, and have every appearance of 

 being complete without this appendage. 



These aspects of the species are shown on Plate ii, figs. 13, 15, 16 



and 17. 



Many of the young individuals, or those in which the earliest develop- 

 ment of cells is perceptible, are enveloped in a sac or sheath, which may 

 perhaps perform some nutritive office in the devolopment of the germs. 



How far the bulb-like appearance at the base of some of the species of 

 Dendrograptus may correspond to the disc or sac of G. hicornis and others, 

 I have not at this time the means of satisfactory determination. 



IV. Ufature and Parts of the Stipe Proper. 



1. The solid axis: 2. The common canal — common body, or coenosarc: 

 3. The calycles or cellules (Jiydrothecd) : 4. Nature and ornaments of 

 the test. 



