Vol. 54.] 



FAUl^A OF THE SKIDDAW SLATES. 



535 



believe, developed as the result of absence of division in the common 

 canal — namely, B. Nicliolsoni, Lapw., and D. afflnis, Nicb. ; these 

 types have much in common, but present slight differences. It is 

 interesting to observe that in both species further development pro- 

 ceeds along similar lines, resulting in the ultimate production of 

 forms with one stipe only ; the one-stiped form developed from 

 D. Nicliolsoni being Azygograptus LapwortM, Nich., * and that 

 developed from D. affinis being Azygograptus suecicus, Moberg. 



There is so great a resemblance between D. ciffinis and Az. suecicus 

 that, but for the abundance of the latter species in the beds and the 

 absence of anything resembling a connecting-canal, it might almost 

 be thought that the presence of only one stipe was merely accidental. 

 In the Azygograpti^ therefore, I consider that we have an extreme 

 type of Dichograptid in which the power of branching is reduced to 

 a minimum, with the result that only one stipe is for Died. 



Didymograptus gracilis, Tornq., may be regarded as a slightly 

 modified form of D. affinis. 



Species. 



Cell-characters, etc. 



No. of 



cells 



to inch. 



Incli- 

 nation. 



Aper- 

 tural 

 angle. 



Over- 

 lap. 



Bryograptus Callavei. 

 Tetragrajptus sp. {Hicksii- 



type). 

 Bidymograptus Nicholsoni 



Azygograptus Lapworthi. 

 Bidymograptus affinis. 



Azygograptus suecicus. 

 Bidymograptus gracilis. 



Slender forms. 

 Cells with outer 

 walls straight 

 y or very slightly 

 curved. Aper- 

 tures approxi- 

 mately straight. 



J 



28 (11) 



26 

 (10-11) 

 20(8) 

 18-20 

 (7-8) 

 18(7) 

 18(7) 



20° 



20° 



20° 

 15°-20° 



15° 

 15° 



105° 



105° 



90° 

 90° ' 



90° 

 90° 



; 



4-i 



GO (2 

 » g- 



13 o 

 O '^ 



P go 



CD ^ 



<S> o 



■ i-s 



(2) Graptolites derived from Clonograptus. 



The mode of development of the more simply-stiped forms from the 

 more complex types is similar to that of the other group — namely, 

 failure of dichotomous division in the common canal. If this be 

 true, we should expect that the most primitive Didymograpti resulting 

 from such development would have their stipes in one and the same 

 straight line : in other words, have apparently resulted from further 

 growth of the so-called funicle, which, as I have shown, should be 

 regarded as consisting of two stipes of the 1st order, since they 

 are known to be celluliferous. Nicholson & Marr seem to regard 

 the angle of divergence between the two stipes of the 2nd order as 

 of phylogenetic importance. I must confess that I fail to appreciate 

 the reason for their view. I am convinced that the Didymograpti in 

 this group have resulted merely from repeated failure in dichotomous 

 division, and I cannot see how the result could have been 

 attained in the unsymmetrical way indicated by those authors. 



