GKAI'TULITES OF NEW YORK. PART 1 637 



tlie. Skiddaw slates D i c h o g r a p s u s a r a n e a and D . s e d g e w i c k i i . 

 We agree Avitli Elles and Wood in considering the first of these names as a 

 synonym of I) . o c t o b r a c h i a t u s , tlie differences being explicable by dif- 

 ferent modes of preservation. 



Hall noticed already that the niiraber of branches is not absolutely 

 unvariable, but that suppression of a dichotomy may result in a smaller 

 number of branches. Elles mentions a septad type, where one branch of 

 the second order fails to divide dichotomously, and a hexad type, where 

 two fail to divide. The only specimen of the hexad type noticed in our 

 material was foimd in the uppermost Deep kill horizon (with 

 D i p 1 o g r a p t u s d e n t a t u s ) , where the Dichograpti are rapidly 

 disappearing. 



Malaise observed a hexabrachiate form in Belgium and proposed for 

 it the specific term D. hexabrachiatus [loc. cit. p.44]. This hexad 

 variation occurs there also associated with octobrachiate forms, and in a high 

 zone Avith D i d 3^ m o g r a p t u s m u r c h i s o n i and D i p 1 o g r a p t u s 

 p r i s t i n i f o r m i s (= d e n t a t u s ) , as in New York. 



These occurrences show that there existed a tendency to reduction, 

 Avhich has finally led to the production of species of Tetragraptus 

 [Introduction, p.567]. 



On the other hand we have observed, in graptolite bed 2, a form with nine 

 branches in which one of the branches of the third order has divided again 

 dichotomously, thus producing two branches of the fourth order [pi. 8, fig. 7]. 

 While this form, by the strict application of the definitions of Logano- 

 graptus and Dichograptus, would have to be referred to the former stage, it 

 is, in the character of the thecae and branches, and its general habit, 

 a D. octobrachiatus. It is evident that hand in hand wath the reduc- 

 tion of the number of branches other changes take place which make a strict 

 application of this one criterion to the separation of the species impossible. 



