70 



BRITISH GRAPTOLITES. 



SrKCinc Characters of Foi;ms nELONOiNf; to the Genus Tetvagraptut 





Horizontal series. 



Dependent series. 



Reclined series. 



Group I. 



Group II. 



Group III. 



Group IV. 



Group V. 



T. quadri- 



hrachiatitf. 



r. cruci- 

 fir. 



T. Ileadi. 



T. Amii. 



T.frHti- 



Pendcnt, 

 flexed 



T. pendens. 



T. ro>- 



llclhii-aifii. 



T. serra. 



T. rPcHna- 

 tits. 



T. Bigshyi. 



Character of stipes . 



Hori- 

 zontal, 

 straight 



Hori- 

 zontal, 

 straight 



Hori- 

 zontal, 

 straight 



Hori- 

 zontal, 

 straight 



Pendent, 

 straight 



Pendent, 



straight 



Reclined, 

 straight 



j 



Reclined, Reclined, 

 straight flexed 



Maximum width of 

 stipes 



2-6 mm. 



4 mm. 



3 mm. 



3— 4mm. 



2 mm. 



0-6 mm. 



16 mm. 



3-2 mm. 



1-6 mm. 



3-2 mm. 



Length of sicula . . 



- 



- 



- 



2 mm. 



2 mm. 



16 mm. 



2 mm. 



2 mm. 



lo mm. 2 mm. 



No. of thccsB in 10 mm. 



10-9 



8 



9-10 



9-10 



7 



9—12 



11—12 



8—9 



12—13 14-13 



Inclination .... 



30'- 40° 



45° 



40° 



45° 



30°-35° 



15°- 20° 



30^ 



45° 



45° 



40°— 70° 



Overlap 



i-§ 



* 



1-1 



-s— T 



-5 



i-i 



i-i 



f-J 



* 



4-1 



Genus SCHIZOGRAPTUS, Nicholson. 



187G. Schizograj)tiit<, Nicholsou, Geol. Mag., dei'. 2, vol. iii, p. 248. 



Folypary bilaterally symmetrical, oonsistiiig of four main uniserial stipes 



produced by dicliotomous division from a primitive Didijmograjytus stage ; 



these four main stipes are rigid, are spread out in a liorizontal plane, and 



give off from one side only (thecal margins) simple lateral ^ branches at 



definite intervals. 



Thecae simple tubes expanding slightly in the direction of their apertures, 



usually in contact for a considerable fraction of their length. 



The polypary is characterised by the possession of stipes of two (dichotomous) 



orders only, but from the stipes of the second order (main stipes) simple lateral 



branches are given off. Thus the Schizograptuti polypary is, from this systematic 



point of view, of the Tetrograptid type ; and may be theoretically regarded as a 



Telragraptus of the quadribrachiatus type, in which the main stipes bear simple 



lateral branches. These lateral branches seem to originate, in all examples known, 



from the ventral or thecal margin of the main stipes. These main stipes increase 



^ Without cutering upon the question as to the structural significance in the distinction between 

 dichotoinous and lateral (monopodial) types of branching, we here employ the term dicliotomous when 

 continuation of growth in the original dii-ection would bisect the angle between the two stipes resulting 

 from division, and lateral when the original stipe continues to grow in the same direction after 

 division as before (Elles, ' Quart. Journ. Geol. Soc.,' 1898, p. 477 ; Euedemann, ' Eep. of State Pal., 

 Now York Stale Mu.seuin,' 1902, p. 584). 



