54 BRITISH GRAPTOLITES. 



first theca." This seems to show that in this species there is a deviation from the 

 normal Diclymograptus type of development, — that is to say, a forecast of the type 

 characteristic of the Diplograptidae ; this, if substantiated, might afford grounds 

 for the retention of this form as the type of a sub-genus (Isograptus, Moberg). 



After the growth of the earlier thecee the normal Didj^mograptus type of 

 development appears to be resumed, each theca of each series developing from 

 the theca next to it on the side nearest the sicula. On these growth-lines can 

 occasionally be detected. 



The apertures of the earliest developed thecas are directed obliquely downward, 

 but subsequently, owing to their curvature, the succeeding ones come to face 

 horizontally outward, and ultimately even upward. The thecae are in contact 

 throughout their length ; in the proximal part of the polypary they are four 

 times as long as wide, but at the distal extremities they are very much shorter 

 in proportion to their width. The apertural angle is commonly 130° after com- 

 pression. 



Affimties. — The species bears a superficial resemblance to certain examples of 

 T ct ra rjrapkis Bigshiji, when two stipes out of the four are hidden. It should, 

 however, be readily distinguished, foi' in T. Bigshiji the polypary is narrowest in 

 its proximal part, whereas in D. <iU)hri'nhis it is widest. This fact was emphasised 

 by Nicholson in his original description ; nevertheless it would appear to have 

 been overlooked by some, as there has been much confusion between the two forms. 



On PI. II, fig. (', we give a drawing of the type specimen of Hopkinson's 

 PhyUo(/irij^fiis nfclhi, from whicli it Avill be seen that this form is identical with 

 B. ijihhernluH. 



Horizon and Localities. — Arenig, Middle Skiddaw Slates (Upper Tetragraptus 

 beds). 



La/,-(>. District : Randal Crag ; White House Fell, Skiddaw ; Bassenthwaite 

 Common. /S7. JJaind's District : Road Uchaf, Ramsey Island. 



Associates, etc. — D. gibbcrnlns occurs associated with Didi/mograptiis hinindo, 

 Azggograptus siiecicus, Tetragraptus qiiadribrachiatus, FJij/Uograptus ti/pus. Good 

 specimens from these beds are in the Woodwardian Museum, the Keswick Museum, 

 the Geological Survey Museum, and in the collection of Postlethwaite, Kes^nck. 

 The type specimens (PL II, figs. 9 a, d) are in the Woodwardian Museum. 



