STUDY OF TEE ORAPTOLITES. 273 



and again dichotomous more than once in most of the species," he does not 

 show that feature in those he has given as the types of the genus ; unless 

 indeed he designates the divisions of the polypary below the cellules as 

 the doubly branching and dichotomous condition, which, if in this case 

 sufficient to constitute a generic distinction, is enough to warrant a farther 

 separation where the branches are repeatedly dichotomous after becoming 

 ceUuliferous. 



In speaking of the monoprionidian forms of graptolites, I have neglected 

 to explain the effects of compression in different directions, and the con- 

 sequent aspect presented by the cellules upon the surfaces of the shaly 

 laminse. This is to some extent illustrated in Plate iii, fig. 24, and in the 

 enlarged parts, figs. 28, 29 and 30. In the extremely dichotomous forms, 

 like fig. 8 a, p. 208, and fig. 29, p. 217, the branches are variously com- 

 pressed, sometimes presenting the ceUuliferous face as a narrow indented 

 surface, limited on each side by the margins of the stipe (fig. 1). We 

 rarely observe the more extremely compressed forms which, when the 

 back of the stipe is presented on the upper side, have 



r r jTi 7 Fig. 1. Fig. 2. Fig. 3. 



w 



\ 



an appearance of double serratures, but with a differ- 

 ent aspect at the indentation of the cellules from 

 true DiPLOGRAPTUs (fig. 2). As these branches become 

 partially turned so as to show the lateral face, we 

 have the aspect presented in fig. 3, where the lower 

 extremity shows some of the cellules marking the two sides, while the 

 upper part shows the true character of the species. 



It is but rarely that the cellules of any of the genera or species are 

 preserved in their proper form and proportions : they are however, some- 

 times found in this condition from being filled with the mineral matter of 

 the surrounding rock ; but often with iron pyrites, which preserves them 

 in their original form more completely perhaps than any other substance. 



In regard to the Genus Phtllograptus, I may not have sufficiently 

 indicated the probable existence of a common body, the evidence of which 

 is obscure, owing to the fact that the cell-partitions extend downwards 

 nearly to the solid axis, and are apparently connected with it at the 

 exterior margins. I suppose that the narrow bases of the cell-partitions 

 are concave towards the axis, and consequently allow the presence of a 

 continuous slender common body, uniting the bases of all the cellules as 

 in ordinary forms of graptolites, where the common body is of variable 

 width, and sometimes very narrow. The general aspect of this type of 

 Graptolite is illustrated in figs. 23 and 24 on page 213, while the figures on 

 Cab. Nat. 35 



