g42 j. nopkinson and c. lapworth on tee graptolites of 



Descriptions of the Species. 



Suborder RHABDOPHORA, Allman. 



Section Graptoloidea, Lapworth. 



Family DICHOGRAPTID^E, Lapworth. 

 Genus Didymograpttjs, M'Coy. 



DlDYMOGRAPTTTS EXTENST7S, Hall, Sp. PI. XXXIII. figS. 1 «-l d. 



1858. Graptolithus ecctensus, Hall, Report of Geol. Surv. of Canada 



for 1857, p. 131. 

 1865. Graptolithus extensus, Hall, Grapt. Quebec Group, p. 80, pi. ii. 



figs. 11-16. 

 1870. Didymograpsus ecctensus, Nich. Ann. & Mag. Nat. Hist. ser. 4, 



vol. v. p. 341, pi. vii. figs. 2, 2a. 



Branches long and slender, becoming gradually wider from their 



origin for the greater part of their length, diverging from each 



other at an angle of 180° ; sicula minute ; hydrothecae about 



24 to the inch, inclined to the axis at an angle of about 40°, 



two or three times as long as wide, very slightly curved, and 



with their outer margins free for one third to one half their 



length; apertures with a truncate margin; apertural angle* 



90° to 120°. 



Our specimens differ from Hall's description only in the angle 



made by the margin of the apertures, which varies considerably 



even in the same individual. But this is probably merely owing 



to varying direction of compression, and Hall's figures show that in 



this respect the British and Canadian forms do not materially differ. 



One result of perhaps compression and cleavage combined is shown 



in fig. 16, in which one branch, near its origin, is seen to have only 



20, while in the other the proportion is nearly 40 thecae to the inch. 



The actual number has here (near the origin of the branches) most 



probably been about 25 or 30 in an inch. 



In an unusually well preserved specimen, consisting of one entire 

 branch with a small portion of the other, the distal extremity is seen 

 to gradually diminish in width to nearly the same extent as the 

 proximal. The specimen is partly preserved in relief, a very rare 

 occurrence in the St.-David's rocks. The presence of the virgula is 

 distinctly indicated by a slight elevation at the dorsal margin of the 

 branch ; and the periderm, again, is well marked and considerably 

 more prominent than the hydrothecae. In a transverse section the 

 hydrothecse would appear to become narrower towards their aper- 

 tures. Where the substance of the Graptolite is removed, the indivi- 

 vidual hydrothecae are even more clearly defined than where they 

 are shown in relief; and their proximal margin, or line of junction 

 * The angle made by the aperture jwith the general axis of the branch. 



