538 



kiss G. L. ELLES ON THE GKAPTOLITE [Aug. 1 898, 



Each primitive Didymograptus whose stipes include between them 

 an angle of 180° may also give rise to other forms by slight modifica- 

 tions. My view is certainly borne out in the examples that I adduce, 

 for in each case the resulting Didymograptiis has its stipes in one 

 and the same straight line. 



(<t) Group containing Dichograptus octonarius, Tet7'agraptus serra^ 

 and Didymograptus arcuatus. 



Nicholson & Marr have already shown the general relationships 

 between these forms (Geol. Mag. 1895) ; it only remains for me to 

 point out the more detailed resemblances in the characters of the 

 cells. 



Species. 



Cell-characters, etc. 



No. of 



cells 



to inch. 



Incli- 

 nation. 



Aper- 

 tural 

 angle. 



120° 



110° 

 110° 



Over- 

 lap. 



Dichograptus octonarius 



(Hall). 

 Tetragraptus serra (Brng.) 

 Didymograptus arcuatus 



(Hall). 



Cells curved, and 

 y with concave 

 apertures. 



24 (9-10) 



24 (9-10) 

 24 (9-10) 



30°-35° 



30° 

 30° 



f 



(/3) Group containing Loganograptus Logani, Didymograptus oeto- 

 brachiatus, D. extensus^ and Tetragraptujs quadribrachiatu^. 



I have indicated in the foregoing pages the lines along which such 

 forms as the above have been evolved, but it is interesting to observe 

 the resemblances of the cells to each other. 



Species. 



Cell-characters, etc. 



No. of 



cells 



to inch. 



Incli- 

 nation. 



Aper- 

 tural 

 angle. 



Over- 

 lap. 



Loganograptus Logani . . . 

 Didymograptus octo- 



brachiatus. 

 Didymograptus extensus. . . 

 Tetragraptus guadri- 



brachiatus. 



1 Cells similar in 

 ! form, curved, 

 j and with concave 

 j apertures. 



24 (9-10) 



20-24 



(8-9) 



24 (9-10) 



24 (9-10) 



35° 

 20°-55° 



40° 

 40° 



90°-95° 

 105°-110° 



110°-115° 

 100° 



3 

 5 

 2 

 3 



2 

 3 

 2 

 3 



(y) Group containing Tetragraptus Headi and Didymograptus 



patulus. 



It seems possible that Tetragraptus Headi may also have been 

 developed from the pentad t5^pe of Didymograptus octobrachiatus, and 

 that from it D. patulus was subsequently evolved. I would also 

 suggest that D. V-fractus may be a modification of D. patulus. 



