BRITISH GRAPTOLITES. 



DESCRIPTION OF GENERA AND SPECIES. 



Famihj DICHOGRAPTID-^, Lapivorth. 



1873. Dichograptidce, Lapw., " Notes on the British Gra|jtolites and their Allies," Geol. Mag., vol. x, 

 p. 555. 



Uniserial Graptoloidea witli bilateral polyparies, bearing simple svibcylinclrical 

 thecEe. Branching' usually dicliotomous, but occasionally irregular. Primary angle 

 of divergence generally 180° or less. 



Genus DIDYMOGRAPTUS, M^Coy. 



1851. Didymograpsus, M'Coy (pars), "British Palseozoic Fossils," p. 9. 



Pohjpary bilaterally symmetrical, consisting of two uniserial stipes which 



diverge from the sicula at angles usually less than 180° (rarely more). 

 Thecae simple subcylindrical tubes expanding slightly towards their apertures, 

 the plane of which is normal ; they are inclined at varioiis angles to the 

 axis of the stipe, and are in contact for a considerable portion of their 

 length. 

 The initial cone-like body (sicula) in Didymograptiis is composed of the usvial 

 two portions — the apertural (or oral) and the apical (or aboral). The apical 

 portion is often seen to be prolonged into the fine initial fhmul {nema) which is 

 believed to be identical with the cirgiola of Diplo<jraptus. 



The sicula occupies the middle of the proximal {initial or central) part of the 

 compound pohjpary (or rhahdosoma), and gives origin to the two stipes (or branches), 

 which usually increase in diameter towards their distal extremities. The stipes are 

 generally straight, l)ut occasionally curved. For purposes of description the form 

 of the curvature is always i-ef erred to the general line of the ventral margin. The 

 ventral angle (the angle subtended l)y the ventral sides of the dorsal walls of the 

 stipes) is selected as the conventional angle of divergence. 



The thecse (cells, cups, or calych's) are arranged upon one side of the stipe only 

 {uniserial). They are not parallel to the axis of the stipe, but are inclined to it 

 at various angles. In form they are simple conical or subcylindrical tubes. They 



