22 J. HopUnson — On a New Genus of Graptolites. 



has, on more than one occasion,^ urged the importance of " defining 

 exactly what is understood by the ' angle of divergence,' since this 

 term," he says, " has been very loosely employed, and has led to a 

 great deal of confusion." This angle he correctly defines as the 

 angle included between the branches on the opposite side of the 

 polypary to the radicle, while the angle formed by the branches on 

 the same side as the radicle, he terms the " radicular angle." This 

 must necessarily in every case be the " defect " of the angle of 

 divergence from four right angles or 360°, and therefore we may 

 here disregard it. It is evident that the correct use of either term 

 depends upon our right conception of the position of the radicle. 

 A perusal of Dr. Nicholson's paper on " Didymograpsus " will show 

 that whatever confusion previously existed upon this point, would, to 

 say the least, be greatly increased if we were to consider the axillary 

 spine, when present, as the true radicle, and when it is not present 

 were to take the proximal spine as the radical; and this is just what 

 he has done. That this latter does represent the radicle, whether 

 the axillary spine is present or not, I have already shown, and thus 

 the determination of the true angle of divergence is rendered certain. 

 In Didymograpsus it is the angle included within the polypiferous 

 margins of the branches ; in Dicellograpsiis and Dicranograptus it is 

 the angle included within the dorsal or non-polypiferous margins.^ 

 In neither genus is it known to exceed two right angles, or 180°. 



We have next to consider the evidence we possess of the con- 

 tinuity of the coenosarc of the two branches. The beautiful symme- 

 try of the branches favours the idea that they were connected at 

 their origin by living tissue ; the continuous or unbroken curve 

 between the branches in the spineless species, points to the same 

 conclusion ; and no dividing septum has been observed. But, in the 

 absence of specimens preserved in the round, we can get no section, 

 and therefore cannot be certain upon this point. In Dicrano- 

 graptus we are equally uncertain, for in two of its species (D. ramosus 

 and D. Glingani) the stem divides acutely into branches, the peri- 

 derm of each branch seemingly being continued through the stem ; 

 while in the other three species {D. sextans, D. formosus, and 

 D. Nicholsoni) the jDcriderm of one branch seems continuous with 

 that of the other, as in Dicellograpsus ; apparently showing that 

 in these species at least the branches are organically connected ; and 

 I believe this will be found to be the case throughout both these 

 genera, and also in their near ally Climacograptus. 



The hydrothecEe are similar in form and structure to those of 

 Dicranograptus, and as these have, in a previous paper,^ been fully 

 described, and shown to agree in all essential points of structure 

 with the hydrothecse of Climacograptus, I will here only draw atten- 

 tion to the fact that these three genera form a distinct and well- 

 marked group of the Graptolite family, differing from all others in 

 the form and structure of their hydrothecae. 



1 Ann. and Mag. Nat. Hist., series iv., vol. iv., p. 240 ; vol. v., p. 338. 

 ^ In Diploffrapsus, Cephalof/rapsus, and Climacograptus, theoretically, the angle of 

 divergence is nil, while the radicular angle is 360°. 

 3 Geological Magazine, Vol. VII., p. 353. 



