DIDYMOGRAPTUS. 4.-> 



wide ; they increase steadily in leiigtli, until they may measure 7 mm., and have a 

 width of 1 mm. The cell walls are curved, so that while the initial angle of 

 inclination is about 85°, the angle which the cell makes with the axis near its aperture 

 is about ^0° ; a lower angle of inclination obtains in the j)i'0.ximal parts of the 

 stipes. 



Affiiiifii'>i. — T). (implns has up to the present time been included in D. Murrltii^oiii, 

 but there can l^e little doubt that it should be separated as a distinct form ; the 

 stipes are never parallel, and the abrupt widening, straight dorsal walls, and 

 enormously long thecse serve to distinguish it from D. Murchisoni and vai-. 

 geminns. Allowing for the distortion to which the rocks in which it occurs have been 

 subjected, it is nevertheless obvious that the distortion cannot have brought about 

 all the features which distinguish it from the species with which it occui-s, for 

 these characters are recognisable in all specimens of this form whatever may be 

 their position in the rock. 



Horizon and Localitij. — Lower Llandeilo ; Abereiddy Bay. 



Associates. — D. amylus is associated with B. Mmxhisoni and var. geminns at 

 Abereiddy Bay, and good specimens are in the collection of the AVoodwardian 

 Museum and those of the Authors. 



We regard the example figured on PI. IV, fig. 3 c, as the type specimen of 

 this species. It is in the Woodwardian Museum. 



Sub-group B. — Type D. indenfns. 



Dependent DidymogTapti in which the width of the stipes remains typically 

 constant throughout their length. 



Didymograptus, cf. indentus (Hall). Plate IV, figs. 4(/ — c 



1858. Ch-aptolithus indentus, Hall, Rep. Greol. Survey, Canada, 1857, j). 128. 



1865. Graptolithzis indentus, Hall, " Grrapt. of Quebec Group," Geol. Surv. Canada, Canadian 



Organic Eeiuaius, dec. 2, p. 74, pi. i, fig. 20. 

 1870. Didymograpsus geminns, Nicholson (pars), Ann. Mag. Nat. Hist. [4], vol. v, p. 346, lig. 6«. 

 1875. Non Didymograptus imlentas, Hopkinson, Quart. Journ. Geol. Soc, vol. xxxi, pi. xxxiii, figs. 



7 a—c. 

 1898. Didymograptus indentus, EUes, Quart. Journ. Geol. Soc, vol. liv, p. 510. 



Stipes from 2'5 to 5 cm. in length, slender, maintaining a fairly constant 

 width of 1'2 mm. throughout; diverging at a primary angle of about 60°, 

 incurving, and then becoming parallel. Sicula slender, tapering. Theca^> 

 eight to ten in 10 mm., inclined at 25° — 80°; three times as long as wide, 

 free one half their lengtli. Apertural margins normal, very sliglitly con- 

 cave, denticles submucronate. 



