162 



BRITISH GRAPTOLITES. 



Fig. 102 c. — Dicellograptus cadi 

 ecus, Lapw. 



Thecie showing extreme intro- 

 torsion. Enlargement of part 

 of PI. XXIII, fiff. 4 c. 



cliitin was of considerable thickness in this species, and 

 it may be suggested that the twisting of the polypary so 

 characteristic of all specimens of IK caduceus was due 

 to the stiff and wire-like nature of the polypary caused in 

 part by this thickened chitinous covering. When the 

 stipes cross more than once, the spaces included by their 

 crossing become larger each time; thus while the first 

 crossing of the stipes usually takes place about '5 — 1 mm. 

 above the initial region, the second point of crossing is 

 1*5 mm. from the first, and the third crossing is still 

 more distant. 



The sicula has a length of 1*5 mm. 



The introversion and introtorsion of the thecae are 



Specific Characters of Forms belonging 





Group I. 





Group II. 







n. complanatui, 





D. ancept. 



D. divaricatm, 







D. intortus. 





var. ornatus. 







var. rigidus. 



var. 

 salopienxis. 







Characters of stipes 



Widely 

 divergent, 



Slightly con- 

 vergent, 



Divergent, 



~ 



- 



Convergent 

 or crossing, 







straight. 



lateral spines 

 conspicuous. 



straight, or 



with slight 



curvature 



distally. 



approxi- 

 mately 

 straight, 



widening 

 rapidly. 



Uniform 



width 



throughout. 





curved. 





Maximum width of stipes 



1 mm. 



7 mm. 



1'2 mm. 



1 mm. 



- 



o mm. 



1 mm. 





Length of sicula .... 



15 mm. 



+ 1 mm. 



13 mm. 



? 



- 



- 



+ 1 mm. 





No. of thecal in 10 mm. . 



10—8 



— 



12—10 



8 



12—10 



14—11 





Characters of thecffi — 

 (a) Free outer wall . 



Straight, 





- 



Approxi- 

 mately 

 straight, 





- 







(/<) Aperture .... 



horizontal, 



slightly 

 introtorted. 







slightly in- 

 troverted and 

 introtorted. 



— 



— 







A.pertural excavation 



Semicircular 



i width of 

 stipe. 





£ width of 

 stipe. 



Wide, deep, 

 and oblique. 



- 









