DIDYMOGRAPTUS. 



Figs. 4 a and b. — Didymogra2)tus 

 extensus (Hall). 









a. Proximal end, obverse view. 

 Outerside, Keswick. Coll. 

 British Museum (Nat.Hist.). 



h. Distal tlieciE of same, com- 

 pressed. 



Figs, -ic and d. — DiJymoijraptus 

 extensjis (Hall). 



horizontal growth of the crossing canal. Consequently the whole appearance of 

 the proximal end is very symmetrical. In addition to this there is a characteristic 

 " widely open " appearance due to the direction of growth of the earliest formed 

 thecae. These seem to be appressed to the sicula for a short distance from their 



origin, but in the region of its aperture they begin 

 to curve steadily away. The walls of these earliest 

 thecae always exhibit slight curvature, but those 

 developed later have their walls approximately 

 straight. 



The thecge are narrow tubes of uniform width, 

 which are three times as long as wide in the adult 

 part of the stipes, but only twice as long as wide 

 near the sicula. The maximum thecal length is 2*1 

 mm. The thecae are free for one third of their 

 length in the distal parts of the stipes, but for nearly 

 as much as a half in the proximal parts ; they have an inclination of about 40^, 

 but this may vary considerably even in the same stipe. In some specimens the 



theca3 resemble in form the peculiar thecae of D. cori- 

 strictus, but the " constrlctiforvt " appearance would 

 seem to be the result of the mode of preservation. 



Affinities. — D. exiensus can readily be distin- 

 guished from most other forms by its slender 

 flexuous character and " widely open " proximal 

 end. From D. nitidus, which it I'esembles more 

 closely than any other form, it can be easily sepa- 

 rated by the smaller number of thecae in a given 

 unit of length. 



Horizon and Localities. — Arenig (Lower Arenig 

 of Hicks), St. David's district; Mytton beds, Shrop- 

 shire ; Middle Skiddaw slates. 



St. David's District : Road Isaf, Road Uchaf , 



Ramsey Island ; north of Talfan, Whitland. Lleyn 



Peninsula : Nant-y-gadwen. Lalce District : Randal Crag, Skiddaw ; below Raven 



Crag, west of Skiddaw ; Outerside ; Knockmurton ; Barf ; Eggbeck. Shropshire : 



Tankerville Mine ; East Gravels Mine. Scotland : Bennane Head, Ballantrae. 



Associates. — In the St. David's district D. extensus occurs in association with 

 D. sparsus, D. iiennatnhis, D. nitidus, etc. Specimens from the English areas are 

 in the Woodwardian Museum, Cambridge. There are examples from the Skiddaw 

 slates in the British Museum (Nat. Hist.) ; and in Lapworth's collection examples 

 from Skiddaw and Shropshire. 



Distal tliecffi in relief, showing " con- 

 strietiform " appearance. Below 

 Eavcn Cray, Skiddaw. Coll. Wood- 

 wardian Museum. 



i^/AH ' 



d. Part of same stipe preserved as a 

 cast, showincf apparent variation in 

 the inclination of the thecae ; con- 

 strictiform appearance no longer 

 observable. 



