STUDY OF THE GRAPTOLITES. 



209 



In other species with a similar general arrangement of parts, the main 

 stipes are frequently bifurcated ; the bifurcations beginning near the 

 base, and continuing as far as the parts can be traced in the stone as in 

 (fig. 8« of Graptolithus {Dichograptus) flexilis , with branchlet enlarged). In 

 some of the species of this character the cellules begin near the base of 

 the stipes, while in one species they are not known to exist except on the 

 outer branchlets. 



Thus far we trace these forms through what appear to be very natural 

 stages in the progress of development of the parts, which are all con- 

 structed upon the same plan, presenting only natural and we may almost 

 say consequent modifications. 



The character of stipes and cellules in all these is such that the sepa- 

 rated fragments would afford no means of indicating whether the part 

 belonged to two, four, or eight-stiped species, or to those with numerous 

 simple stipes, or with branching stipes, unless the fragment retained a 

 bifurcation. It is on this account that I have not adopted several pro- 

 posed designations for the graptolites with a single series of cells, since our 

 collections are mostly in a fragmentary condition, which must render it 

 impracticable with our present knowledge, to make the proper reference. 



A variety of form is exhibited in the division termed Dendrograptus, 

 in which we may conceive of the numerous stipes near the base becom- 

 ing conjoined into one strong stem, with the bifurcating branchlets 

 spreading above, and this stem probably fixed in the soil. We then have 

 a representation of the typical forms of this genus, as in the accompany- 



FlG. 9. 



Fig. 10, 



ing figure of Dendrograptus flexiiosus (fig. 9, branchlets enlarged) and as 

 illustrated on Plate iv of this memoir. 



In the species Dendrograptus hallianus (figure 10), as shown in the 

 enlargements of the branchlets, we have a form of cellule similar to that 

 Cab. Nat. 27 



