Vol. 54.] PATINA OF THE SKLDDAW SLATES. 475 



Occurrence. — In the Middle Skiddaw Slates, associated with 

 Dichograptus octobrachiatus. 



Localities. — Barf; Randal Crag, Skiddaw ; Slape Crags, above 

 Hope Gill, Brackenthwaite ; Carlside Edge ; Bassenthwaite Sand- 

 beds. 



Genus Loganograptus (Hall) modified. 



The description of the genus Loganograptus given by Hall ran as 

 follows : — 



' Polypary consisting of more than eight simple stipes proceeding 

 from a single axis, with a distinct broad corneous disc' (See 

 ' Introd. to the Study of Graptolitidee,' 20fch Ann. Eep. N.T. State 

 Cab. Nat. Hist. 1867, p. ^26.) 



This definition requires modification, for it would seem to imply 

 that the presence of a disc is essential, and that the number of 

 stipes is sufficient ground for separating the form from Dicho- 

 graptus. This does not seem to be the case : — 



(1) Porms are known precisely similar in every detail, with 

 the exception of the disc. It seems likely, therefore, either 

 that the disc has perished during preservation, or was only 

 developed to meet special needs, and consequently its presence 

 is not of generic value. 



(2) The number of stipes is not an important distinction, but 

 rather the capacity in Loganograptus for greater dichotomy. 



A complete Dichograptus, as, for example, a typical D. octo- 

 brachiatus (Hall), is a form in which dichotomy can occur only twice 

 in all. In Loganograptus, on the other hand, dichotomy may take 

 place three, four, or more times. In both genera it always takes 

 place near the base. 



The number of stipes varies with the completeness of the dicho- 

 tomy ; a form may have only six stipes, and yet be referable to the 

 genus Loganograptus, as I shall show. I am not aware that a 

 greater number of stipes than thirty-two has ever been recorded. 

 The various other numbers that are met with are the result of failure 

 of complete dichotomy. A specimen before me has only seven stipes, 

 and yet it must be referred to Loganograptus, since it shows a greater 

 capacity for dichotomy than is ever shown by Dichograptus. In 

 this form all four stipes of the 2nd order are developed, but only 

 two of these undergo further division, so that only four stipes of 

 the 3rd order are produced ; of these only one undergoes further 

 division. The number, therefore, is made up as follows: — 



2ii-)-3"i +21^=7. 



Similarly, it is possible to have a Loganograptus with only six 

 stipes ; in this case three out of the four stipes of the 2nd order 

 persist as single stipes ; the fourth divides dichotomously, giving rise 



