452 



Dr. 0. Herrmann — Disfribittion of Grapiolites. 



In the first place it must be noted that the presence or absence of 

 this so-called axillary spine cannot at all aifect the decision of the 

 question whether we have or have not to regard it as the sicula, as 

 Bot only this process, but also the middle one of the three processes 

 placed side by side, is sometimes to be found, sometimes not. This 

 is shown by Nicholson's figure ^ of D. sextans, Hall. 



But if we compare the figures here given (Figs. 2-6) of repre- 

 sentatives of various genera, the decision of the question before us 

 seems to proceed directly from them. DiceUogra'ptus, Hopk., occupies 

 a middle position between Didymograptus, M'Coy, and Dicranograptus, 

 Hall ; it is placed in the immediate vicinity of the latter genus, and 

 both are referred to the same family. In Didymograptus the sicula is 

 quite visible, and no doubt is possible as to its position; in Dicrano- 

 graptus the lower part of the two branches has grown together, and 

 the sicula is thus imbedded. In the first case the pointed end of the 

 sicula is actually upon the dorsal side of the branches, theoretically 

 also in the second. In Dicellograptns, Hopk., the same position is 

 occupied by the " axillary spine." This is consequently the true sicida. 



Fig. 2. Didymograptus vacillans, Tullb., Fhyllograptus shales, JSTorway. Nat. size. 



Original. 

 „ 3. Dicelhgraptus Forchhammeri, Gain., Ireland. Nat. size. After Lapworth. 

 ,, 4. iJicranograptus ramosus, HslII, Hudson Eiver Group, Canada. Nat. size. 



After Hall. 

 „ 5. Fiplogjaptus angustifoliiis, H., Ireland. Nat. size. After Lapworth. 

 ,, 6. 7!/o«o(7rflji?i^e<s ^re^«»i«<«, Lapw., Upper Silurian, Scotland. Enlarged. After 



Lapworth. 



We find a still greater diiference of opinion when we endeavour 

 to make out what the various authors understand by the angle of 

 divergence. Not only do the individual naturalists support different 

 views, but even one and the same author defines the idea of the angle 

 in question diiferently in different parts of his writings. Hopkinson, 

 for example,^ in 1871, explains with regard to the genus Didymo- 

 grapttis, M'Coy, that the angle of divergence is the •' angle included 

 within the polypiferous margins of the branches " ; but on a subse- 

 quent occasion ^ he says that in the genus Didymograptus, M'Coy, 



1 Monograph of the British Graptolitidse, p. 63. 



2 " On Dicelhgraptus^' Geol. Mag. Vol. VIII. (1871), p. 22. 



3 " On the Graptolites of the Arenig and Llandeilo Eocks of St. David's," Quart. 

 Joum. Geol. Soc. vol. xxxi. (1875), p. 640. 



