188 



TWENTIETH REPORT ON THE STATE CABINET. 



Fi2. 29. 



Graptolithus milbsi (Dichograpius, Salter). . 



These barren, or non-celluliferous portions of the graptolitic body, are 

 not otherwise essentially different from other parts of the stipe. In the 

 absence of cellules, they are consequently more cylindrical, and apparently 

 more solid, as if the test were thicker and the interior canal less developed 

 than in the other parts of its extent. 



3. The Central 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 laminae, 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 (Gr. 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 

 absorption. 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 development of the germs. 



