STUDY OF THE GRAPTOLITES. 203 



side of a solid axis, it is easy to perceive how the germ of an analogous 

 form may develop from its initial point two series of cellules upon a stipe, 

 where the parts diverge in opposite directions from the common origin or 

 initial point. One step further in this direction will give us the four-stipcd 

 forms, where the germ of the common body, with its additional elements 

 of subdivision, produces the quadripartite frond ; and so onwards, until 

 we have the numerously-branched fronds and the branching stipes. 



In all these, the germ in its incipient development will differ very little. 

 It may consist of the radicle or initial point, with the solid axis and the 

 common body separated into two, four, eight, or an indefinite number of 

 divisions, each one bearing its solid axis and common canal. These sub- 

 divisions sometimes all take place near the origin, which is always central ; 

 and the divisions continue simple throughout, or do not bifurcate after 

 they commence to develop cellules. In others the stipes are again divided, 

 and this subdivision is only limited by the extent of the frond. In all 

 these fronds the parts are always arranged symmetrically or bilaterally on 

 the two sides of the initial point, as has been illustrated in the preceding 

 pages. 



In the greater part of the monoprionidian forms, we have only modifica- 

 tions or extensions of the simplest form of development shown in figures 

 8 and 9 of Plate iii. Where the divisions at the base become more nume- 

 rous, as in figures 15, 16, 20 and 22, it is simply a farther subdivision of 

 the stipes, but all taking place near the initial point. In a further deve- 

 lopment, the subdivision takes place at any point along the branches, either 

 near to, or distant from the initial point. Where the divisions at the base 

 become more numerous (and indeed in some of the four-stiped species), 

 we often find a thick corneous test, of the same substance as the 

 other parts of the graptolite, uniting the bases of the stipes and continuing 

 along their margins. This disc has a greater or less development, not 

 always corresponding to the size or extent of the stipes. It is sometimes 

 absent, apparently from accident, and some of the four-stiped species are 

 not known to possess it ; while it has never been observed in any of the 

 species where the stipes are properly branched, or divided iu the celluli- 

 ferous parts of their length.* 



The interior of this corneous disc, previously described as apparently 

 composed of two plates of the test, has probably been occupied by some 

 softer substance, which may have been an extension of the common body, 

 or have had in some degree the character of the common body of the stipe. 



The development of the diprioiiidian forms, as deduced from the young 

 graptolites which we find associated with the reproductive cell-bearing 

 stipes, would show that these forms of graptolites exist as single and 

 simple independent stipes from the commencement of their growth. 

 Nevertheless I conceive that both Retiolites and Retiograptus may have 



* Sec remarks upon the Genus Dichoguaptus under Generic descriptions in this paper. 



