22 



BRITISH GRAPTOLITES. 



Fig. 12. — Didymograptiis euodus, Lapworth. 



S's-.v ^■>>-^\'v^ '' y^' ;/^i r'y\ y 



Distal theca?. Enlargement of part of PI. I 

 fiff. 10 h. 



Ijiit unfortunatelv all other details regarding the structure of the proximal end are 

 obscure in the few specimens as yet known to us. 



The tliecas show slight douljle curvatm^e of 

 their outer walls, and the concave aperture is 

 frequently prolonged into a conspicuous denticle. 

 The distal tliecae are inclined at an angle of 30° 

 to the general direction of the stipe, but in the 

 region of the sicula the inclination does not 

 exceed 20°. They number seven to eight in 

 U) mm. The overlap increases from mere contact near the sicula to a maximum 

 of one half the total length in the more distal portions of the stipes. The 

 individual thecse may measure as much as 3"2 mm. in tlie later parts of the 

 stipe, but are much shorter near the proximal end. 



Affodties. — The species may be distinguished from all other Didymograpti by 

 the remote cells and the almost imperceptible increase in width of the stipes ; in 

 some respects it approaches in character the members of the ]J. affinis group, and 

 in others to members of the D. extensus group ; it may, in fact, be regarded as 

 forming a link between these and the group in which it is included. 



Associates, etc. — It occurs associated with B. Murchisovi, D. superstes, and 

 certain Diplograptidaj. Lapworth originally cited S. Scotland as a region where it 

 occurred in association with JJ. superstes, l)ut this is an error ; it appears to be as 

 yet unknown from Scotch deposits. 



Horizon and Localities. — Lower Llandeilo. 



8t. David's District : Abereiddy Bay (south end) ; Llanvirn Quarry ? 

 A few specimens are in the Woodwardiaii Museum, Cambridge, including the 

 type (PL I, figs. 10 a, h) originally described and figured by Lapworth. 



Characters of the Group of D. 



superstes. 





D. superstes. 







Flexuous 



D. euodus. 



1 

 Form of stipe . 



Horizontal 



Width .... 



3 mm., 



24 mm., 



Length of sicula 



1 



increase rapid 

 11 mm. 



increase very 

 gradual 

 1 05 mm. 



Origin of stij^es 



Suboral 



Suboral 



No. of thecse in 10 mm. . 



9—10 



8 



Inclination 



40° 



30° 



Overlap .... 



i-f 



1 



1 



