20 BRITISH GRAPTOLITES. 



120°. Tliecse nine to ten in 10 mm., inclined at a fairly large angle, four 

 times as long as wide, and with increasing overlap towards the distal parts 

 of the stipes. Apertm'al margins normal, concave, produced into a broad 

 triano'ular denticle. 

 Description. — The stipes at their origin measure only 6"8 mm., but they widen 

 regularly and quickly within the first 7 cm. to 3 mm. They alwaj'S show curvature 

 of some kind, the commonest form being that in which the stipes are slightly con- 

 cave near the sicula, then convex. They become horizontal towards their distal 

 extremities, and are known to attain a length of at least 25 cm. 



The sicula is small and inconspicuous (1*1 mm.). It is slender near the apex, but 

 widens very rapidly in the direction of its aperture. It is placed somewhat obliquely 



with reference to the stipes, and remains of the nema 



Fig. 11 a. — Didymograptus superstes, • n i i , , i t <■ •, 



Lapworth. cau occasioually be detected proceedmg trom its 



^^iV^ apical extremity. The first theca seems to originate 



'^3-\K'~'^^^^i^S^ midway between the apex and apertiire of the sicula 



Proximal end, obverse view. Enlarge- OU the left side (obverSC \new). The thcCa 1^ is 



unusually broad at its origin with the sicula ; possibly 

 this is the expression of an attempt to render the attachment of the stipe to the 

 sicula more secure. This form therefore exhibits a change from the more usual 

 method of securing permanent attachment to the sicula by appression of the earliest 



developed thecse ; in this case theca 1^ grows away 



Fig. 116. — Didymonraptiis superstes, Lap- p .■, • i , ^ • l^ • i c 



worth. irom the sicula at once, leavmg the sicula tree on 



■^N^ the left side for a short distance above its aper- 



\-, ture. In the reverse view of the polypary the 



)\ crossing canal is seen to be oblique, and to grow 



,n '^ ^ in such a manner that part of theca 1 conceals a 



'^ considerable amount of the apertural region of the 





iA""'' ■^ 



sicula ; subsequently the tliec?e develop normally. 



.'>^v {'v'^'^ ""Y- ^ The thecge themselves are simple tubes, ex- 



'^'^h'-'.A ' ^^ ^^k ^ panding slightly in the direction of their auer- 



< i '■'''':' \ J / '"^^^^ tures. When fully developed they are three times 

 ^v^^'y'""' ^ V\ as long as wide, and have a length of 3-2 mm. 

 ' They are inclined at a niaxinmni anole of 40° but 



Specimen showing the nema, the proximal _ _ ' 



end being concealed by a film. Tottie- near the sicula the angle is usually not more than 



hams Burn, Castle Douglas, Kirciid- q 



brightshire. Coll. H.M. Geological 25 . I lie VClltral Wall IS sllglltlv COUCaVC, and tllC 



Siu-vey, Edinburgh. , i i • 'i 



apertural angle being normal varies A^atli the 

 angles of inclination of the thecae. The tliecse number nine to ten in 10 mm. at all 

 points along the stipes. Near the sicula they are only just in contact with each 

 other, but in the distal region of the stipes they may overlap for as much as one 

 half to two thirds their leng-th. 



