Graptolites. 291 



sacs a young specimen just beyond the margin and barely sepa- 

 rated from the fibres of the sac (Fig. 8), and another below the 

 sac. It has been recently supposed that numerous pedicellate 

 bodies (Fig. 9) found in the graptolitic shales of Dumfriesshire 

 are ovigerous vesicles, but as yet no case has been observed in 

 which a relationship could be certainly traced between the 

 fossil and the graptolites, and consequently the true nature of 

 these bodies is very doubtful. 



The first developed hydrothecae are generally smaller than 

 those formed later, making the outline of the polypary taper 

 towards the older portion. In the genus Graptolithus, the 

 earlier cells are sometimes more slender, and more separate 

 from each other than they are afterwards. When the perfect 

 ■ size and form is attained, the additional hydrothecae are exact 

 repetitions of each other, and the polypary is parallel sided. In 

 a few cases the newer as well as the older cells are small, as in 

 Phyllograptus (Fig. 5 a) and in D. folium, His., in which the 

 depth of the cells is increased by layers added to the mouth. 



EXPLANATION OF PLATE. 



Fig. 1. Graptolithus priodon. View showing the mouths 

 of the hydrothec33. 



Fig. 2. Back view of ditto, showing the slender, solid axis. 



Fig. 3. The disc and portion of the branches of Dicho- 

 grapsus. 



Fig. 4. Climacog raphes scalaris. a, showing the cell open- 

 ings in front; h, showing a portion of the second series of 

 openings. 



Fig. 13. Showing both series, preserved at right angles to 

 that shown at Fig. 4. 



Fig. 5. Phyllograptus ilicifolius. b, ideal section, showing 

 the four series of cells. From Hall. 



Fig. 6. Diplograpsus pristis. Specimen from which the 

 reproductive sacs are supposed to have been removed (by 

 maceration), leaving the marginal fibres by which they were 

 attached. 



Fig. 7. Young state of the same species. 



Fig. 8. Diplograpsus, figured by Hall. 



Fig. 9. Pedicellate bodies referred to in text. 



Fig. 10. Diplograpsus tricornis, showing different stages in 

 its growth between its adult condition, 10 a, and its youngest 

 state, 10 d. 



Fig. 11. Four cells and a polypite of the recent Sertu- 

 larian, Dynamena pumila. 



Fig. 12. Retiolites Geinitzianus. A fragment magnified. 



Fig. 14. Graptolithus convolutus, ditto. 



