STUDY OF THE GRAPTOLITES. 237 



of its meaning and its limits. In the figure first given by Mr. Salter 

 in the Geologist (1861), where the name was proposed, wc have a properly 

 branching form, not very dissimilar from G. milesi of this paper, but 

 without a disc ; and no allusion is made to a disc. In 1863,* Mr. Salter 

 gives a figure of Dichograpsus aranea of Salter, a form with eight 

 simple celluliferous stipes; and also "fig. 10, Dichograpsus (Salter), 

 with its corneous cup (from Logan)." Fig. 11, Dichograpsus sedg- 

 wicki is another species with eight simple stipes. At that time, it does 

 not appear that Mr. Salter had seen a British specimen with a central 

 corneous disc ; nor do I know that any one has yet been illustrated. The 

 addition of the disc to Dichograpsus was a subsequent idea, and the 

 adoption of such forms as G. aranea and G. sedgwicki as the types of his 

 genus by Mr. Salter, leaves out the really branching forms, like the 

 diagram in the Geologist for which the name was first proposed. If sepa- 

 ration is to be made on such grounds as these, a farther one must be adopted ; 

 and the Genus Dichograpsus, which now includes three types, must be 

 restricted to the types D. aranea and D. sedgwicki. This arrangement 

 will leave those forms with the central corneous discs to form a new genus, 

 for which I propose the name Loganograptus. Those which are repeat- 

 edly dichotomous, like G. Jlexilis, will constitute a third genus ; for 

 notwithstanding that Mr. Salter says the Dichograpsus "is doubly 

 "branched and again dichotomous more than once in most of the species," 

 he does not show it 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 

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

 separation where the branches are repeatedly dichotomous after becoming 

 celluliferous. 



In speaking of the monoprionidian forms of graptolites, I have neg- 

 lected to explain the effects of compression in different directions, and the 

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

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

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

 like fig. 9, p. 176, and fig. 29, p. 188, the branches are variously com- 

 pressed, sometimes presenting' the celluliferous 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 an appearance of 

 double serratures, but with a different 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 pre- 

 sented in fig. 3, where the lower extremity shows some of the cellules 



* Quart. Jour. Geol. Society Vol. xix, p. 137. 



