78 BRITISH GRAPTOLITES. 



Stipes of first and second oi'ders very sliort; main sti})es of the tJiird order 

 typically straight and rigid, often several centimetres in length, slender at 

 origin, bnt widening qnickly to a maximnm breadth of o*2 mm. Theca? five 

 to ten in 10 mm., inclhied 20° — 55°; onter walls slightly cnrved, four times 

 as long as wide, in contact one half to one third their length ; apertnral 

 margins concave, normal. 



Description. — The stipes of the first order are about 1 mm. in length, those of 

 the second order slightly longer — 1*6 mm., while those of the third order may attain 

 a length of from 12 to 15 cm. Many young examples of this species barely attain 

 2"5 cm. in length, and the stipes are relatively narrower. The main stipes, while 

 typically straight, are often preserved showing a cei'tain amount of curvature. 



It is only on the main stipes that any details regarding the thecfe can be 

 determined, though these are undoubtedly present on the stipes of both first and 

 second orders. The thecge on the main stipes make an angle of about 20° with the 

 axis of the stipe at their bases, but these curve so decidedly that the inclination 

 rises to 55° near their apertures. The thecee when mature have an average length 

 of 8—4 mm., but in smaller forms they are much shorter. 



The whole proximal part of the polypary is frequently envelojied in a mem- 

 branous disc, which varies greatly in size in different individuals. This disc, how- 

 ever, may be altogether wanting. 



Remarks. — Bicltonraptus odohracluatus, although typically eight-stiped, exhibits 

 great variation in the number of its stipes. The typical form, having eight stipes 

 of the third order, appears to be the commonest, and from this form the species 

 derives its name. But individuals, agreeing in all respects except in the number of 

 stipes, have seven, six, or five stipes, and there is no reason for doubting that they all 

 belong to one and the same species. In the normal type with eight stipes of the 

 third order there are four short stipes of the second order, and two short stipes of 

 the first order ; both stipes of the first order and all four of the second order 

 undergo dichotomous division. 



In the form with seven terninial sti})es (septad type) both stipes of the first 

 order divide dichotomously, but one of the stipes of the second order fails to do 

 this, and persists as a single stipe, the other three dividing as in the normal form. 

 In the form with six terminal stipes (hexad type) two of the stipes of the 

 second order show no dichotomous division, and in the pentad type (five stipes) 

 only one stipe out of four divides dichotomously. These variations of Dicho- 

 graptus octuhrachiafns are b(^st regarded as ahorted specimens of the ty])ical octad 

 form. 



Total failure of dichotomy in the stij)es of the second order would obviously 

 result in the production of a 'Ih'tnKjruptus of the qiiadribrachiatiis type. 



Affinities. — D. odohrachiatiis (including var. Sedgtoichii) may readily be dis- 

 tinguislied from the only other species of Dlchograptus as yet recorded, D. separaius, 



