532 



MISS Gr. L. ELLES ON THE GRAPTOLITE [Aug. 1 898, 



eight stipes, forms with seven, six, and five stipes which seem to be 

 identical in all respects except the number of branches. 



Then Tetragraptus is reached. A further reduction in the powers of 

 dichotomy would result in a Didymograptus of the extensus-ty^Q. 



The artificiality of a system of classification that includes in the 

 same genus two forms having such totally different lines of descent 

 as the above forms of Tetragraptus is perfectly obvious, but it is 

 not my purpose here to suggest a change in this respect. 



In dealing with the phylogeny of the Skiddaw Slate graptolites 

 I divide them into two large groups : — (I) Those derived from a 

 Bryograptus-ioTin. ; (2) those derived from a Glonograptus-ioxWi. 



Nicholson & Marr regard the angle of divergence as being of 

 phylogenetic value. In the foregoing pages I have called attention to 

 the fact that conditions of preservation must affect this angle within 

 certain limits, especially if we hold Lap worth's view of the mode of 

 life of the graptolites ^ ; but given a graptolite with a certain amount 

 of rigidity, the angle will probably be approximately constant. Also 

 I think that it has a certain value when used in a general sense, 

 and when absolute constancy in numerical values is not insisted on. 

 For example, I do not believe that a Tetragraptus of the fruticosus- 

 type would ever resemble a Tetragraptus of the quadribrachiatus- 

 type, but I consider it very likely that great variation might be 

 observed in a delicate Clonograptus-ioicm in the angles at which 

 the stipes of various orders were inclined to each other. For, if we 

 consider the rhabdosoma to have been attached to some floating 

 body, and its branches flexuous, it is obvious that these branches 

 might ultimately come to rest in very various positions. 



(1) Graptolites derived eeom Bryograptus. 



(a) Group containing Bryograptus ramosus var. cumbrensis^ 

 Tetragraptus pendens^ and Didymograptus indentus. 



I have already referred to the lines along which Didymograptus 

 indentus has been evolved from Bryograptus ramosus var. cumbrensis 

 through Tetragraptus pendens ; sl detailed examination of these forms 

 shows how complete is the resemblance between them. 



Species. 



Cell-characters, etc. 



No. of 



cells 



to inch. 



Incli- 

 nation. 



Aper- 

 tural 

 angle. 



Over- 

 lap. 



Bryograptus ramosus var. 

 cumbrensis, 



Tetragraptus pendens. 

 Didymograptus indentus. 



/'Stipes similar in 

 1 width. Cells very 

 j slightly curved, 

 ' and veith slightly 

 concave aper- 

 tures. 



24 

 (9-10)2 



20-24 



(8-9) 



25 (10) 



20^ 



15-20° 



30° 



115°- 

 120° 



120° 

 115° 



less ^ 

 less \ 

 abt. \ 



^ See Walther, Zeitschr. deutsch. geol. Gesellsch. vol. xlix (1897) p. 235. 

 ^ Figures in parentheses in this and the foUovring tables denote the number 

 of thec£e in 10 mm. 



