500 



MISS G. L. ELLES ON THE GKAPTOLITE [Aug. 1 898, 



other known graptolite is exceedingly abundant in parts of the 

 Skiddaw Slates. In 1868 Nicholson observed the form, and noticed 

 that it was not in all particulars identical with Hall's species. He 

 stated that the English specimens were more closely related to the 

 type that Hall had descrihed and figured as a young form, and was 

 inclined to think that they should be regarded as a distinct variety. 

 Young forms of the species are certainly very abundant in the 

 Skiddaw Slates, but there are also adult forms, and these seem to be 

 very closely related to Hall's type. There are still certain minor 

 differences, but these seem to me to be so unimportant that I cannot 

 discern any necessity to distinguish them from the typical form 

 under a varietal name. 



Hall gives the angle of divergence as 175^; in our Skiddaw Slate 

 forms it is often very much lower, the initial angle of divergence 

 being in some cases only 130^. How far this is the result of 

 preservation I am not prepared to say, but I am inclined to think 

 that even in these stouter and more rigid graptolites the angle of 

 divergence may be affected within certain limits, though hardly 

 perhaps enough to account for the above discrepancy. 



In the typical American form the stipes appear to proceed at 

 once horizontally or xevy nearly so, but our English specimens show 

 a greater variation in form. In some the stipes are directed down- 

 wards, in others horizontally or even upwards. The English species 

 has also rather fewer thec« in a given unit of length, twenty-eight 

 to thirty-two in 1 inch (eleven to thirteen in 10 mm.), instead of 

 thirty-two to thirty-four in 1 inch (thirteen to fourteen in 10 mm.) ; 

 the greatest length observed in any stipe is 3 inches (76 mm.). 



Notes on Structure. — The sicula is fairly conspicuous; it is 

 usually about -^ inch (1-58 mm.) in length, and is very slender in 



Fig. 19. — Didymograptus nitidus ; from 

 Randal Crag, coll. Woodwardian Museum. 



its apical part. It is 

 often placed obliquely 

 with regard to the 

 stipes. The earliest 

 theca (th. 1^) arises 

 from the sicula at 

 some distance above 

 the aperture on the left 

 side. The connecting- 

 canal is fairly oblique ; 

 it gives rise to the 



earliest theca of the ^ -. 



secondary stipe (th. 1") ^ 



in the normal manner. In some specimens the stipes diverge from 

 each other at their origin at an angle of about 175°, their dorsal 

 walls being straight ; in others there would seem to be a tendency 

 to grow at first downwards, the angle of divergence being about 

 130° ; in exceptional cases this growth may be continued, but more 

 frequently it is arrested after the development of six to ten thecae on 

 each stipe, the stipes then assuming the horizontal position. In this 

 case the dorsal walls are at first curved, but subsequently straight. 

 In other cases again, where the downward growth is arrested, 



