DIDYMOGRAPTUS. 23 



Declined Series. 



Didymograpti with approximately straight stipes diverging downward, in 

 which the ultimate angle of divergence is typically less than 180°. 



Group IIT. — Type D. affiuis. 



Didymograpti in which the stipes are narrow, of approximately equal width, 

 growing more or less rigidly outward and downward, and originating slightly 

 above the aperture of the sicida. Thecse are inclined at small angles, and the 

 amount of overlap is never great. 



Didymograptus affinis, Nicholson. Plate II, figs. 1 a, h. 



1869. Didymograpsus affinis, Nicholson, Aun. Mag. Nat. Hist. [4], vol. iv, jj. 240, pi. xi, fig. 20. 



1870. Didymograpsus affinis, Nicholson, Ann. Mag. Nat. Hist. [4], vol. v, p. 343, fig. 4. 



1875. Non Didymograidus affinis, Hopkinson, Quart. Journ. Geol. Soc, vol. xxxi, p. 645, pi. xxxiii, 



figs. 6 a — c. 

 1898. Didymograptus affinis, Elles, Quart. Journ. Geol. Soc, vol. liv, p. 503. 



Stipes 12 to 18 mm. in length, very narrow througliout, never exceeding '6 mm. 

 in width, diverging from a small sicula at angles of 90^ to 1 50°. Thecae 

 seven to ten in 10 mm., outer walls straight, three to four times as 

 long as wide, free for the greater part of their length ; inclination 15° 

 to 20°. Apertural margins straight, perpendicular. 



Description. — The stipes have a uniform width 

 YiG.na.-md^nograptns affinis, .g ^^ .3 ^^^^^^ ^^^^^ ^^^ commonly 12 to 18 mm. in 



^^A^ Itjngth. 



:;;^i^'^'^^7^ fpj-^g sicula is minute, being about "8 mm. in 



Proximal end, reverse view; enlarge- ig^^^tli ; the first theca (th. 1') Originates at a very 



ment of part of PI. II, fig. 1 ?>. » ' \ / a j 



short distance above tlie aperture of the sicula and 

 grows immediately outward, so that the sicula is free on one side of its aperture 

 for a small fraction of its length. In the reverse aspect of the polypary the crossing 



canal is seen to be narrow, oblique, and short, and it 

 Fio. 13 b.-Didjp>wgmptus affinis, ^Q^ceals part of the aperture of the sicula. 



The general characters of the thecae are simple ; 

 they are tubes which widen gradually in the direc- 

 Distal tbec^^; «£i=»J-j»ent of part of ^-^^^ ^f ^^^leiv apcrtures, and have a length of about 



1*5 mm. Nicholson gives the number of thecae as 

 seven in 10 mm., but in some specimens this number would appear to be exceeded. 



