Yol. 54.] FAUJfA OF THE SKIDD AW SLATES. 473 



{a) Cloi^ograptus flexilis (Hall). 



1858. Graptolithus fiexilis, Hall, Geol. Surv, Canada Rep. 1857, p. 119. 

 1865. Graptolithus flexilis. Hall, ' Grapt. of the Quebec Group,' Geol. Surv. Canada, 

 dec. 2, p. 103 & pi. X, figs. 3-9. 



1873. Clonograptus flexilis, Nich. Ann. Mag. Nat. Hist. ser. 4, vol. xi, p. 138. 



A. slab in the British Museum (Nat. Hist.) shows three specimens 

 referable to this species, and two of these are exceedingly good.^ 



The dichotomy takes place at fairly distant intervals, the distance 

 increasing with remoteness from the base. The larger of the two 

 good specimens has seventeen ultimate stipes, but these are by 

 no means all of the same order. 



(1) From the lower right-hand stipe of the 2nd order, four 

 branches of the 5th order and two of the 4th order arise as 

 the result of repeated dichotomous division.^ 



(2) Erom the upper right-hand stipe of the 2nd order only 

 two stipes of the 5th order are developed ; the others are of 

 the 4th order, and show no signs of further division. 



(3) Prom the upper left-hand stipe of the 2nd order no 

 branches of a higher order than 4 are developed, and these 

 persist as long single stipes. 



(4) From the lower left-hand stipe of the 2nd order only two 

 long stipes of the 3rd order are developed. 



This may be graphically represented thus : — 



4v^2iv_j_2V4.3iV4.4iv_^2"i=6^+9i^4-2"i=17. 



The formula for a typical Olonograptus should be 



There has been therefore considerable atrophy. 



In the smaller of the two specimens there are only thirteen 

 ultimate stipes, and none of an order higher than 4 : — 



(fl) Order 2 gives rise by dichotomous division to four stipes of order 4=4^^, 

 (jb) Order 2 ,, ,, „ two stipes of order 4 and one of 



order 3=2^^+1"^. 

 (c) Order 2 „ „ „ two stipes of order 3=:2iir. 



(c^) Order 2 „ „■ „ four stipes of order 4= 41^^. 



Total: 10^^+3"^=13. 



The stipes are all slender and flexuous ; they increase in width 

 from their origin up to a maximum of -Jj inch (1-05 mm.). 



The thecse are clearly seen on several of the stipes, and on stipes 



^ [Since writing the above I have had reason to doubt that these specimens 

 came from the Lake District ; but as the species unquestionably occurs at Barf, 

 the description is allowed to stand.] 



^ Tr^e so-called ' funicle ' is regarded as consisting of two stipes of the Ist 

 order, in accordance with recent work. Compare the description of Tetra- 

 grobptus quadribrachiatus (Hall), p. 485. 



